Make the boot floppy by going to the "K" menu -> Configuration -> Boot and Init -> DrakFloppy. Put a disk in your floppy drive, check the defaults, and make the boot disk. Test it to see if it works. When you re-install Windows, it will overwrite your MBR. Can't change that. If you don't move any partitions around or create new ones, your lilo or grub menus ought to be fine. Once Windows is installed, boot with the Mandrake boot floppy, get a root commandline prompt and, for grub, do:
# cd /boot/grub
# sh install.sh
for lilo, do:
I think that ought to do it. If I'm wrong, maybe someone else can help out here or maybe you could do a search for re-installing grub/lilo at Google Groups.