It is always recommended to install Windows first and then Linux, however if you would prefer [for some weird reasons] to do the opposite then follow this guide!
You need a Linux LIVE CD. E.g.-> ubuntu or suse or mandriva,etc etc.
Boot from your Live CD. Now open a terminal and type in:
Now it will show you the correct root partition.
root (hd0,1) [replace 0 and 1 according to the output!]
sudo grub-install /dev/sda [or hda , whichever is your MBR]