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!

Requirement:

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:

grub

find /boot/grub/stage1

Now it will show you the correct root partition.

root (hd0,1)   [replace 0 and 1 according to the output!]

setup (hd0)

quit

sudo grub-install /dev/sda [or hda , whichever is your MBR]

reboot

Done!