Boot windows on a second HDD.
grub.conf:
title Windows
map (hd0) (hd1)
map (hd1) (hd0)
rootnoverify (hd0,0)
makeactive
chainloader +1
GRUB cannot boot DOS or Windows directly, so you must chain-load them (see Chain-loading). However, their boot loaders have some critical deficiencies, so it may not work to just chain-load them. To overcome the problems, GRUB provides you with two helper functions.
If you have installed DOS (or Windows) on a non-first hard disk, you have to use the disk swapping technique, because that OS cannot boot from any disks but the first one. The workaround used in GRUB is the command map (see map), like this:
grub> map (hd0) (hd1) grub> map (hd1) (hd0)
This performs a virtual swap between your first and second hard drive.
See: