Restoring GRUB boot menu from ubuntu live CD.

If windows overwrites your MBR you can boot into any ubuntu live CD and do the following from a terminal window. This happened to me today and the steps below solved my problem.

sudo grub
this will log you into GRUB, you should get a grub> prompt.

find /boot/grub/stage1
This will search for the location where grub is installed and will return a string of the form hd(?,?) which will be used in the next command.

root hd(?,?)
This will tell GRUB the correct location. Do not forget to replace the ? will the values returned frol the previous comand.

setup (hd0)
This will install GRUB on your MBR.