http://www.troubleshooters.com/linux/grub/specialboots.htm

Once booted, assuming Windows is on /dev/hda4, add the following to your /boot/grub/menu.lst:

# Windows
title  Windows
map (hd0,0) (hd0,3)
map (hd0,3) (hd0,0)
rootnoverify (hd0,3)
chainloader +1