Do you already have XP Installed on the computer? i.e. was the system already dual booting with XP and Linux, and now you just want to get rid of Linux?
Fedora core uses grub, which writes in MBR. I've had this problem before myself, and the only way to get rid of grub is to reformat the MBR partition. First you need to get a hold of rescue floppy disk. Then, you use fdisk to execute the following command:
That's it. just press Ctrl+Alt+Delete, Windows XP should be starting up.
Let me know if this doesn't help