Theoretically it could be done.
You would have to find a Linux based AV suite that runs on the Pi and connect the PC hard disk via the Pi USB and hope the AV programs understand the Windows programs (disk structure is not an issue).
Should only take 5 / 6 times as long as booting from the manufacturers boot packages via USB on the PC and may trash the program structure
If you want to help your customers with the Pi then I would look at setting up a NAS for backups, VPN to access their own network and banks etc from public hotspots, maybe a TOR Router 'black box' for safer surfing and start to teach them to protect themselves and the family. You could even set up a mail filter using SPAM Assassin to sit infant of their ISP mail host.
Need Pi spray - these things are breeding in my house...