|They do have a virtual floppy either for xp or vista that many people use.|
As above everyone has made cd's and usb images that might be loaded on usb's.
What virus do you have in MBR?
Why not replace the mbr using the xp disk?
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, Make an autorun.inf folder on all usb drives.