Hi Raistlin666, Since we aren't sure of the configuration, and boot files, I would reccommend that you download a version of "gparted live" @
http://download.tuxfamily.org/gpartedlive/, this is a live cd where you will be able to partition, and set flags on your drive as well as erase the partition you want . Set as boot , or many other options.
Always install windows first, then the Linux distro, was it Mandriva that you installed , just pay attention to the partitioning, and know which one is what .( would be my guess if the partition tables got mess up) Or Ubuntu?