This link will give you a clue:
http://www.linuxloader.com/modules.p...&pid=18&page=5
when you see your partitions when doing "custom partitioning" during the install you will have a blue partition showing, that is your windows partition. Don't play with that one. One easy way to do this would be click on each partition shown (except the blue windows one) and delete each one then hit the "autoallocate" key and let Mandrake choose partitioning structure( does it out of the unpartitioned space).
Alternatively, resize your partitions and assign mount points to them with same tool. (Again stay away from windows partition). To do this, simply click on one of the non blue partitions, you will want at least three non windows partitions: (/ , swap, & /home)
Just a suggestion, I would resize the first partition to the right of windows to say 3 gigs and use mount point button to assign it as /, and the next partition I would resize to 500 to 700 megs and assign the mount point swap, then I would resize remaining or create partition to use all remaining space and assign it /home.
Remember if you fudge there is an undo button there. Changes are not committed till you hit the done button.