what virus do you know?
cause i know the conficker jumps through networks and usb's so it could be that it was on a usb that upluged in after formatting and it hopped over then?
I'm wondering if it was a rootkit? I think, ( not positive) you may have to do an MBR fix to get rid of a rootkit. I'm not sure if a reformat will do the job completely... anyone know for sure?