I've been using an AVG Rescue CD to boot Windows systems from a CD, rather than the copy of Windows installed on the system's hard drive, and then perform an antivirus scan of the system. The AVG Rescue CD provides a Windows GUI for performing scans and I've found it works very well. The cost is currently $149.95 in U.S. dollars.
Searching for other rescue CD's, I also found one from F-Secure, which uses a Knoppix LiveCD to boot a system to perform an antivirus scan of the system. You can use it to boot a Windows system to check the system for viruses without booting into a possibly infected copy of the Windows operating system. F-Secure Rescue CD 2.00 is free and can update itself over the network, if a DHCP server is available on the network to provide it with IP configuration information. You don't need to understand Linux to use the software; you are presented with prompts to walk you through the process of scanning a system.
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