Well mandriva has diskdrake, which is an easy to use GUI disk partitioning tool. You just need to know which partition is C:/ and which is D:/. You just have to click on D:/ and format it to any linux fs you like eg. ext3, reiserfs, xfs, jfs.
Well you can do one thing. Go to disc management in windows and delete the D:/, don't partition it with disk management. Then mandriva will detect it as empty partiton and you can select.
BTW, how much space is there on the D:/. 5GB will be enough if you gonna use linux as secondary OS.
And if your C:/ is NTFS make a small partitition of 2-3 GB as FAT32, which can be read and written by both OS'es.