If it's genuine, you should have some way to restore the system at least to manufacturer's settings with thier own restore disc rather than an XP one - if you don't have such a disc, then you're in a genuine pickle, since the usual way to fix that is to restore the file from the XP install disc (using the recovery console), or reinstall XP

Other unlikely causes could be a corrupt boot.ini file or hard disk partition table, but again, an XP CD (or some other bootable CD like Knoppix) would be the way to fix it.

I'd suggest find a friend nearby that can lend you a copy of their CD (make sure it's the same service pack) to try to repair it, or download one and do a clean install