Definitely back up your stuff........try I think its called partition magic to shrink current windows partition,not sure if that can be done with NFS or not.If so, you wont have to reinstall windows.Just be warned, windows filesystems typically spread stuff all over a drive.......not contiguous like linux.
Not sure how mandrake does it but you will probably have to make a linux and linux swap partition, after that mandrake should install with just a few prompts and questions.LILO or grub should be easliy set up to give a boot choice at system start.......with slackware linux it does that on its own usually.