Just to make sure: you now boot using GRUB, which is invoked from MBR, don't you?
I know nothing about FC, but currently boot many partition/systems with GRUB with no problems. I would just back up current /boot/grub/menu.lst just in case the installer doesn't recognize all your systems from current menu. Then all you have to do is to restore the section booting WindowsXP to your new menu.lst and it spares you from searching man grub
As long as you do not damage your XP partition there is nothing to be afraid of.