If the domain is not a true domain then this problem occurs. Its like if you use your localhost to run magento store,then you will face this problem.
You can overcome this by browsing using ur ip address or you can remove these three line from the following page.
app\code\core\Mage\Core\Model\Session\Abstract\varien.php
$this->getCookie()->getDomain(),
$this->getCookie()->isSecure(),
$this->getCookie()->getHttponly()
Comment these lines line (may be at line 78…);
Hope magento admin will work now on your localhost with the following url. 
http://localhost/magento/admin/
Just try out and play with your stores……
Posted by Gyan on June 4, 2009 at 12:02 pm
Hey thanks, have been struggling these two days to get the solution. It finally worked! cheers
Posted by sasi kumar.c on June 5, 2009 at 10:59 am
HI milansaha,
Thanks a lot buddy.. its working now… i hv struggled past 3 days in this problem.. feel very happy..
Thank u so much
Posted by abudis on July 3, 2009 at 1:08 am
Hi….
thank you so much…..
Posted by Kilroy on July 6, 2009 at 2:42 pm
Thanks a bunch.
It now works correctly (though it is slow as hell locally)
This should really be mentioned in guides on how to install Magento on Wampserver
Posted by Nasir on July 11, 2009 at 3:43 am
hi
dear
How are you. I want to talk you
regards
nasir
Posted by Milan on August 16, 2009 at 4:18 pm
@ nasir….u can leave your queries here and i will reply you.
You can also reach me : milan.saha@gmail.com
Posted by vlad on July 8, 2009 at 2:26 pm
You are the man!
Posted by Nasir on July 11, 2009 at 3:41 am
Thanks a lot to give me my login problem sonve. 12 hours trying
Posted by milansaha on July 11, 2009 at 5:15 am
Hello…all of u are welcome !
@ nasir….u can leave your queries here and i will reply you.
You can also reach me : milan.saha@gmail.com
Posted by Jeremy on July 18, 2009 at 7:10 pm
Hey, thanks for this. I found safari worked fine with no modification, but firefox would not work without altering the session_set_cookie_params as suggested.
In 1.3.2.2 I had to comment all lines of those params from line 77 to 84. If I comment just the (3) suggested I got a blank screen. Just a heads up.
Thanks for adding to the community. I am new to this so you are my saving grace.
Posted by Avijit on July 21, 2009 at 12:30 pm
Ok, In 1.3.2.2 I had just comment out those three lines that you have told me and it’s worked for me. Thanks for your suggestion. Oh, by the way I am using xampp, Windows XP
Posted by tasid on July 27, 2009 at 10:06 pm
thank youy very much for your nice & working solution.
simply a awosome solution.
again thanks a lot.
Posted by milansaha on July 31, 2009 at 11:45 am
Nice to know. You are welcome.
Posted by Aurelio on July 28, 2009 at 6:52 am
Thanks, I tried to replace localhost by 127.0.0.1 and it’s now working.
I just installed: 1.3.2.3 in WAMP and if you change the locahost “name” then you will be able to log in Admin Zone.
Posted by milansaha on July 31, 2009 at 11:45 am
Yes…that is also possible because the default IP to browse the apache server on a localhost.
Posted by brillie on July 31, 2009 at 8:32 am
Many thanks…!!!
Posted by Tushar on August 4, 2009 at 5:54 pm
Thankx Man
It works
Posted by Vinny on August 6, 2009 at 8:23 am
Thanks a lot milansaha !! Its saved my time..
Many tanks !
Posted by csemrm on August 11, 2009 at 3:57 pm
if i this 3 line off then ,the following error shown
“There has been an error processing your request.”
Posted by milansaha on August 11, 2009 at 5:38 pm
you might forgot to remove a coma before these line.
Posted by thabetinfo on August 14, 2009 at 8:54 am
thankssssssss
Posted by csemrm on August 15, 2009 at 1:40 pm
Hello brother
I got another solution for installation in local PC and access admin panel.
For access admin panel only replace
http://localhost/magento ->http://192.168.0.1[Local IP]/magento .
thanks
Posted by Milan on August 15, 2009 at 3:37 pm
That might be working if that IP is assigned to your pc.
On the other hand by default the IP for any apache server has an IP and that it is 127.0.0.1
So you can also try with http://127.0.0.1/magento
Posted by Backus on August 16, 2009 at 6:57 am
Fucking a, thank you so much, what a stupid bug.
Posted by Jez on August 26, 2009 at 9:08 pm
Thanks… had me stumped for ages!
Posted by Rojan John v on September 8, 2009 at 7:41 am
HI milansaha,
Thanks a lot….. its working now… i hv struggled 2 hours in this problem.. feel very happy..keep in touch
Thank u so much
Posted by Rojan John v on September 8, 2009 at 7:46 am
{{base_url}} is not recommended to use in a production environment to declare the Base Unsecure Url / Base Secure Url. It is highly recommended to change this value in your Magento configuration.
What will do ?
Posted by Milan on September 8, 2009 at 8:15 am
If you are working on a true hosting i.e. if you have a domain name(www.abc.com) for your site then you just enter that url in this field instead of {{base_url}} .
Thats it.
Posted by Saswata on September 9, 2009 at 6:39 am
Thank u very very much.It works finally.It saves me lots of headache.
Posted by Sheeba on September 16, 2009 at 8:29 am
Thank you very much…..Are u from the magento team?
Posted by milansaha on September 16, 2009 at 8:34 am
You are welcome. I am not from magento team.
I am just a developer from Dhaka,Bangladesh.
Posted by evoratec on September 16, 2009 at 8:37 am
Thanks a lot, man.
Posted by Sheeba on September 16, 2009 at 10:58 am
hi,one more help is needed.
When we add new files/modules in magento,it is not detecting the changes.How to overcome this problem.
Posted by milansaha on September 16, 2009 at 11:16 am
It depends on changing to many files. So tough to guess what the exact problem is there.
Posted by Raju on September 18, 2009 at 7:23 pm
hi.. great man… simply super…
Posted by Raju on September 18, 2009 at 7:30 pm
one more help milan..
how can i integrate hellopod themes..
here i downloaded full version of hellopod themes..
they gave instruction also..
regards that instruction we need to replace app and skin folders within the root folder..
i replaced that folder.. but it showing error to me..
can u help me pls
Posted by Milan on September 18, 2009 at 7:55 pm
I guess it should work if you place the files properly.
Then you have to write the theme name form admin area for your store to see in action.
If you still face problems then i think there are problems with the xml file for the theme to make the proper template structure in magento layout.
Posted by Raju on September 19, 2009 at 8:09 am
Thanks for imm reply…
let me check again properly… thanks
Posted by Kulk on September 19, 2009 at 12:56 am
Thanks my friend
Posted by Sophia on September 21, 2009 at 9:46 pm
Thanks man, i battled with this stuff for 2days.
Posted by Yogesh on October 8, 2009 at 8:23 am
Hey, Thanks a lot, I tried it by setting a enter 127.0.0.1 http://www.localhost.com it also works but if you click on any link it automatically redirects to http://localhost/magento instead of http://www.localhost.com/magento.
But your solution is perfect
Posted by Yogesh on October 8, 2009 at 8:33 am
Hi, I have one problem, I need to integrate Indian payment gateway (times of money) in Magento. could you please guide me in how to create module for this? or please tell me where I can find the files of authorized or paypals payment gateway file it may help me to create new module.
Posted by M Hussain on October 12, 2009 at 11:34 am
Thanks for you tips. Its very useful for new user.
Posted by supunmali on October 15, 2009 at 3:35 am
Really useful.
Posted by supunmali on October 15, 2009 at 4:32 am
I am havin this issue, that I did add the modern theme. But now i cannot change back to default theme, i did get when I first installed magento. How can I get it? Even I cannot change to another theme. It gives errors in the front end view.
Why is it? I did change Default in Design to default get the default theme. (For modern theme I changed it to Modern) after adding the extension.
Posted by milansaha on October 16, 2009 at 3:33 pm
I guess the problem may occur due to improper configuration from the admin area or for any other error in the page.xml file.Firstly you need to make sure that you placed all the files for the new theme in the proper directories and then you have to enter the theme name from the admin area.
Thats it. Your theme should be in action.
Posted by supunmali on October 21, 2009 at 6:57 am
Thanks a lot.. It worked.. had to change the page setup and after deleting the cache it worked.
Posted by Bedanta on October 15, 2009 at 6:15 am
Good one, this fixed the issue in a jiffy
These are precisely the lines one needs to comment in the file varien.php (the ones starting with // are commented). Also remember to take out the comma from the end of the line $this->getCookie()->getPath()
session_set_cookie_params(
$this->getCookie()->getLifetime(),
$this->getCookie()->getPath()
//$this->getCookie()->getDomain(),
//$this->getCookie()->isSecure(),
//$this->getCookie()->getHttponly()
);
Thanks
Posted by Pablo Veintimilla on October 24, 2009 at 4:44 pm
Work, thanks!
Posted by Abdelkarim on October 26, 2009 at 10:30 pm
Wow its work
thanks alot
Posted by Error en instalación de magento: No se puede iniciar sesión como administrador después de la instalación | Teofilo Israel on November 8, 2009 at 7:30 pm
[...] Vía [...]
Posted by phpini on November 13, 2009 at 6:55 am
thanks lot
Posted by George on November 19, 2009 at 3:08 pm
My setup stop working after configuration page. I can not get to “Create Admin account” page.
Last url in browser:
http://127.0.0.1/index.php/install/wizard/configPost/
The error is:
HTTP 500 Internal server error
Any ideas?
George
Posted by Arun on November 20, 2009 at 6:50 am
Hi,
Many thanks for this. Now its working fine. Just a warning, which tells about base_url. But this doesn’t seems to be a problem. Need to take care of this if I’m moving to production mode. Now I’m just setting it up locally.
many thanks
Regards,
Arun Suresh.
Posted by Aster on November 20, 2009 at 10:50 am
Go check this site for a detailed version of this article. http://markuzweb.blogspot.com/2009/08/cant-login-to-magento-admin-panel-after.html <– This blog contains a mouthful of tips which comes very handy as well.
Posted by Johnba on November 20, 2009 at 12:55 pm
Thanks a lot, it works fine!