Saw this in a security list and thought it was worth posting. It is a CD/DVD you can burn to test a machine while the main OS isn’t running so any real bad virus type critters on the machine can’t block the check. Best part is that it is free. It is Linux based but that is fine for scanning your Windows machine’s disks.
http://www.free-av.com/en/tools/12/avira_antivir_rescue_system.html
F-Secure also provides one of these for free.
http://www.f-secure.com/en_EMEA/security/security-lab/tools-and-services/rescue-cd/index.html
It is nice because it’s based on Knoppix, which has wide familiarity, and because it can either fetch pattern updates live over the Internet, or on a truly offline machine, you can load the patterns onto a USB stick and use the boot CD and the USB stick to scan with the fresh updates.