Dual Boot:
In a two hard drive setup;
gedit /boot/grub/grub.conf
Change time out to a value like:
timeout=5
To give you time to change OS
Then add this to the bottom of the page:
title Windoz map (hd0) (hd1) map (hd1) (hd0) rootnoverify (hd1,0) chainloader +1
In a single drive system you can just do this:
title Windows rootnoverify (hd0,0) chainloader +1