Look for The Virus post on this forum marked "Virus FBI Computer is Locked" this post Has answers with it that give you a link to get antivirus rescue disk for Free.

However use another computer to download and burn the file to CD/DVD or maybe copy to USB

You will need to press "F2" or "F10" to enter the boot menu and change the CD/DVD drive/USB to the first boot device

Note this CD/DVD/USB will run when computer boots up and before windows boots so you could log out and restart the computer.

Note: when Windows is not Running Viruses\Malware cannot run because their never Mini Operating Systems just Programs