Cannot login to anything on the internet!

Firstly, I know it's ironic that I'm able to log in to this site, but it worked through "post a question" and nowhere else. I just hope that after typing this it would actually work, but anyway, here's my problem:

-I cannot log into anything on the internet with a browser. I have tried my email, forums, game sites, anything.

-When I type in the correct password (I'm 120% sure it's right) it would accept my log-in and then refer me back to the log in page as if I haven't logged in yet

I have tried:
-both http and https (both works)
-four browsers: Opera, FireFox, Chrome, IE7 (does not work on all four)
-virus scans, malware scans (nothing showed up)
-closing all non-essential processes.
-making sure that my browsers are accepting cookies
-resetting router to default settings
-system restore

nothing works!

I am using Windows XP SP3.

Any help on this would be greatly appreciated. I need to check my email!

-Whatsthis