Say you had a laptop that was set up to dual-boot windows and linux. And then, say, for some unknown reason, that the windows partition got completely hosed. So, after reinstalling windows on that partition, how do you fix the MBR so that grub runs to give the option of starting either windows or linux?
1. Boot into rescue mode from linux install disks
linux rescue
2. Let the partition be mounted under /mnt/sysimage
3. After mounted, change the root to /mnt/sysimage
chroot /mnt/sysimage
4. Reinstall grub
grub-install /dev/sda
Or use /dev/hda or whatever the main disk on the system is