GRand Unified Bootloader is a Linux-world boot loader, though it can boot any OS you can fit on a drive. Its great for choosing between Windows and Linux on dual boot machines and choosing between different kernels on a given Linux system… you could even use it for booting between different Windows instances.
Anyhow… your machine’s boot sector has croaked.. what to do?!
Load up the distro’s rescue CD and mount the boot partition, say its device is /dev/sda1 ( ‘sda’ means ‘first SCSI device’, the ’1′ means ‘the first partition’)
# mkdir /boot
# mount /dev/sda1 /boot
Now startup grub and get to work:
grub> root (hd0,0)
grub> find /grub/stage1
grub> setup (hd0)
This basically means: Your root parition is on the first disk’s first partition, and under the /boot directory is the grub information to place into the MBR, and then to actually write it there.
This will make more sense once you do it. Or not.