not sure if the reinstallation for windows xp is an actual os disk or just something that accesses a recovery partition on your computer. Let's hope it's the former. Insert the disk into your cddrive, and reboot your computer. You should be prompted to boot from the cd, do so. Then choose to do a recovery and get to a console. type fixmbr, it should work it's magic. reboot, taking out the cd and you should be good. search the forums for more specific info.

if the cd isn't an actual os cd but something that accesses a recovery partition, you might have some other options, but everything i'm thinking of uses linux, so probably not the best option. good luck.