Virus Rescue Disk

If your computer is completely unusable due to a virus infection, then you need to reboot with another operating complete with built-in antivirus. Many of the antivirus companies have created this a quick package to download. You just need access to another computer to put the Rescue Disk onto a USB stick or CD. See:
Kaspersky Rescue Disk
Sophos Bootable Anti-Virus
Bitdefender Rescue CD
AVG Rescue CD