You could scan with a bootable cd like Kaspersky or Dr Web.
The only downside is that whatever viruses are removed and if they infected system files, the machine may not boot correctly afterwards-- resulting in a repair of windows.
http://devbuilds.kaspersky-labs.com/devbuilds/RescueDisk/http://www.freedrweb.com/livecdhttp://michaelstevenstech.com/XPrepairinstall.htm
