carbonel

Generally, what I do is take the hard disk out of the computer and temporarily attach it to a second computer to run an AV program. (There are a number of cheap hardware devices that make it easy to plug a hard disk into a USB port on a second computer. Here's one (http://accessories.us.dell.com/sna/products/External_Hard_Drives/productdetail.aspx?c=us&l=en&s=bsd&cs=04&sku=A1164717).)

If you think it's less trouble to keep the disk in the computer and try to fix the operating system from there, that's sometimes possible, but it can get even geekier. This (http://www.nu2.nu/pebuilder/) is a utility that lets you make a Live CD version of Windows, which you can boot and run an AV program from. I'm guessing you might find it easier to pay someone or find a geek friend to temporarily lift the hard disk out of the desktop computer.

Doing an upgrade in place over the existing Windows OS (or installing a second minimal version of Windows) on the infected disk are iffier propositions. If the virus is clever, it may quickly knock out the new version you install, and more stuff can get corrupted. FWIW, if you want to geek out, I gave pecunium some tips that might be useful, last year, in an ML thread (http://nielsenhayden.com/makinglight/archives/010900.html).