If you want specific advice about your particular situation, I would suggest you boot into Fedora and log in as root to run the command: fdisk -l (lower case Letter L) and post the output here. This command shows the partition information for your computer. It's not difficult to re-install Grub and there are many posts here at LQ explaining it.
what are the risks on installing Windows XP into a Drive with Fedora already in it
One thing it will do is overwrite your mbr, won't ask or tell you it's being done. But that's easily overcome re-installing Grub. Make sure you know which partitions have Fedora on before you do the install of xp so you don't write to the wrong partition.