There are many cases in which either you, your friends, or your customers may have a computer with a virus, but no active protection from an Anti-Virus program. In such cases, you can generally use Trend Micro’s Housecall, found at http://housecall.trendmicro.com from either normal Windows or “Safe Mode with Networking” (to get into safe mode, for most XP / 2000, press F8 just after the BIOS screen and before the Windows splash screen).

From Housecall, you can generally identify the virus(es), which you can then look up in their virus database and get removal instructions for. Housecall also has a Anti-Spyware feature, so it can be dual-purposed when you have a spyware and virus attack.