That suggests that you have both set as master or both as slave - to get both drives to work (be recognised by the BIOS) you need to have one as master and one as slave (jumpers on the back of the hdd)
then set the linux drive as the master so that the machine boots linux, then load DrakBoot in the Mandrake Control Center and tell us which boot manager it says you're using.
If you're using Grub (better) then edit /boot/grub/grub.conf (you will need to be root) and add the lines
(at the beginning) and
Code:
title Windows
root (hd1,0)
chainloader +1
at the end
Then when you reboot you will get a menu for 10 seconds that lets you select which OS to boot into.
If you're using Lilo then someone else can tell you which file to edit