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 :p
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 :p