Windows is installed in bootcamp, right?
Autocad and other autodesk apps use a very intrusive antipiracy system that writes to an empty space between the MBR and the adjacent partition.
http://forums.autodesk.com/t5/AutoCAD-2011/AutoCAD-2011-modify-Master-Boot-Record/td-p/3006074
There's no easy way to get rid of this except raw editing the sector or completely reformat the drive, if that's the reason why you can't install, I'm sorry. Personally, I think autodesk has gone too far.
Last edited: Dec 15, 2012