i would put the WinXP drive as slave and add the following to your grub.conf file
map (hd0) (hd1)
map (hd1) (hd0)
In my computer I have a similar situation to yours. I run Fedora Core 3 on my primary disk and WinXP on my slave disk. So my guess is that if you connect the WinXP drive as slave and add what I have above, it should work.