This is for Windows (XP or Vista) dual boot with Fedora Core (any version)
Boot the Fedora cd disk #1 (installation disk).
At the installation menu, type in linux rescue (hit the enter key)
Now system will try to find your linux installation and all goes fine it will mount your root filesystem under /mnt/sysimage directory.
At the terminal prompt, Type in chroot /mnt/sysimage
then type: cd /boot/grub this is where your grub files are.
Type cat grub.conf and note which hd it is referring to.
If this does not work for some reason, type in df or rdev and this will show you where the root drive is, usually the first entry (line).
Then type; (I am installing grub on the MBR of first hardisk)
This may take a minute or so and should just return a command prompt with no errors. Once it does, type in reboot and see if grub comes up normally.