You'll need to use your distro's first CD (usually, that's the bootable one) and get into some kind of rescue console. With FC3, this can be done by inserting the CD#1 and starting up the system, or rebooting from XP. Then, at a certain point in the FC3 install cycle, you type
and a limited shell comes up with your Linux partition already mounted. You then change to this partition's /boot directory (if this is on a seperate partition for you, I don't know exactly what you'll do - mine is all on one physical partition) You then startup grub's console by doing
at the commandline. Then, to get the capability back to boot on choice any of my operating systems (either XP or FC3) I do at the grub console:
grub> root (hd0,1)
grub> setup (hd0)
And then I quit and reboot. When my system starts up afterward, I have the grub screen back and can choose which operating system to use...
Hope this helped!