One of my favorite tools, as you can see from previous columns, is Knoppix. The great thing about this Linux distribution is that you don't have to know anything about Linux in order to be able to use it.
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Once you can get Knoppix to boot, go into the menu and look for the anti-virus software that is installed. Depending on the version of Knoppix that you have, it could be Clam AntiVirus or something similiar. Run the anti-virus software and before doing anything get the latest signature files downloaded. After the signature files are downloaded, run a file level clean on the computer. Depending on how infected your computer is, this could take a while. Once this completes, remove the Knoppix cd from the drive and reboot the computer, allowing to go into Windows. Try at least one other anti-virus software package and make sure that it comes up clean with no viruses being reported.