Why browser keep asking for username and password no matter what website?

When attempting to go to any website, a pop up window would show and say something like “The server http://websitename requires authentication….”, and below it there is the username and password field for you to type in.
If I click on cancel, it would give a 404 error or ‘cannot access resources’, depending one what browser I use.
It’s this way with IE8, Chrome, and Firefox. I’ve scanned the pc with malwarebytes and avg 2011 free a few times. First couple of times it found viruses/malware and I kept deleting them and rescanning till no other viruses/malwares were found.
Anyone have any idea?