What is shown when you select XP? If its just a boot problem, try booting from XP setup disc and go to Recovery Console. After login, type "fixboot" (without quotes) and press enter. If everything is successful, next time you will be able to boot into both the OS.
You can also try "fixmbr" but that will remove GRUB and thus you will not be able to boot into Fedora (also if problem with XP is somewhere else, you will not be able to boot in either of them). You will need Fedora's installtion disc (live CD or DVD) to re-install GRUB. Here's a
link to do that (its for Ubuntu, but it can be applied for Fedora).
If re-installing XP is necessary, the same method of re-installing GRUB can be used. In either case you need not require to re-install Fedora.