I think that's an oxymoron - Microsoft Antispyware.
Needless to say, don't log into anything you need to put a password into. Changing a password when you have a keylogger installed will just give the person who keylogged you the new password.
Boot into safe mode (with network support) and update your anti-virus signatures. Then run a complete scan with your AV software and hope it catches the culprit. It's not a sure bet that all AV packages will catch all keyloggers though, so download and install a good anti-spyware program (Ad Aware or Spybot S&D) and run a full scan with that as well. Then delete that M$ antispyware software.
Hope this helps