Maybe something is wrong with your partition table, so Mandrake cannot use it. Use other partitioning tools, like bootitng. It is free for trying and can resize ntfs partitions without data loss! So, maybe you can download it at http://www.terabyteunlimited.com
, install it, and use it. First, if you have windows, resize the windows partition so that it leaves at least 2GB for linux install. Than, make a ext3 partition about 65% of your free space, then add a /home partition for your data, about 20% of your free space, and a swap partition that's twice the size of your RAM. Then go on and install mandrake.