I have XP and Mandrake 8.2 on my system. I loaded XP first then loaded Mandrake, it now boots with lilo and i can choose which OS to start, they both work fine. I do not know why you would want/need to use the XP boot loader rather than lilo, lilo works fine for me.
Make sure if you are using a USB keyboard or mouse that you remove them and use PS2 items or your Mandrake install with come to a screeching halt a little over halfway through!! Once Mandrake is loaded you can then boot into KDE (don't know how to change it on other desktops) and use the Mandrake Control Center to change your mouse back to USB then reboot and all should be fine.
I have now had Mandrake 8.2 on my system with the nVidia drivers loaded and everything working great for almost five full days and i have not screwed it up!! A record for me. And believe me i have been playing with it (another way of saying sub-consiosly trying to break it!!) and it has been great. I prefer it to XP so far although i am having difficulty getting Counter-Strike to work under Wine, but i'll get it going soon enough.
Doh!!