I think you're going to have to provide a slight-bit more information about your setup...

The easiest (?) way I can think of is to make two separate partitions... one for winblows and another for Mandrake. It's probably easiest to install Windows on the first partition and then install Mandrake on the second (Windows won't necessarily/easily respect the other partition and/or OS but Mandrake will accomodate fairly easily). You'll then want to use one of the dual-boot programs that come with Mandrake, such as LILO, to allow you to boot in to either operating system.

The other scenario: you could install Mandrake on the entire disk, then obtain something like VMWare (an O/S emulator) for Linux and install Windows underneath that... overall, it tends to keep Windows from crashing the entire system. Side-effect: VMware will run you about three bills (US$).

Similiar to the previous: install VMWare for Windows and install Linux under that... though I'm not sure how well it works, etc. I also haven't heard of anyone doing it...

There /used/ to be an option to install Linux under a FAT/NTFS filesystem (presumeably on Windows) and then use something like "loadlin" to run Linux. I've not heard of anyone doing that recently and I'm not sure how well it works (or even if it's an option anymore).