You can try booting off the Windows XP cd, and going in to the recovery console. When you get to the command prompt try typing 'fixboot c:' or 'fix boot c:'(assuming the boot loader is on C) And that should remove GRUB. If it doesnt help, im afraid all you can do is reformat, or put FC3 back on as you said before. When I needed to uninstall FC3/GRUB I did 'fixmbr' and it fixed it fine, also 'fixboot c:' worked for me when I was trying to uninstall GRUB from one of my machines.
Anyway, good luck.