joined a mp3 downloading site, but can`t remember which 1, and not in my history help?

Answer 1

look in your cookies folder (C:\Documents and Settings\YOURLOGONNAME\Cookies) as this should hold some clues to websites you've authenticated to.

Answer 2

First answer is good. but here's a suggestion:
WRITE IT DOWN!!!!
Name of web site as www.?????????
User login name
Password.
Get yourself a notebook and pen. When you are happy with the site you can use automatic log-in in Firefox or IE7, but you still need the written info after a re-format