1. Create boot disk.
For Fedora Core 3:
/sbin/grub-install /dev/fd0
For Fedora Core 4+:
mkbootdisk `uname -r`
2. Reinstall Windows.
3. boot with floppy then
grub-install /dev/hdb
Rescue CD
1. Boot using FC3 CD then
linux rescue chroot /mnt/sysimage grub-install /dev/hdb
http://www.fedoraforum.org/forum/showthread.php?t=1272