Try one of these: http://www.techmixer.com/free-bootable-antivirus-rescue-cds-download-list/
Make the CD on a different machine and then boot to the CD on the suspect computer and let it run. If you need data from the drive (assuming no backup was made) then also try this:
http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
Disregard the Vista mentioned in the page, as it will work with any Windows OS. You could also give EasyBCD a try: http://download.cnet.com/EasyBCD/3000-2094_4-10556865.html
If there is a virus after all this, then wipe the drive with DBan : http://download.cnet.com/Darik-s-Boot-and-Nuke-for-CD-and-DVD/3000-2094_4-10151762.html and reinstall Windows 7.