A tool which you can use to see what is taking up space on a given partitition/drive: Tree Size Free at http://www.jam-software.com/freeware/index.shtml
If you have enabled System Restore, that takes up space.
If you have a program like Go Back installed, it takes up space with backups.
If you have any Symantec program installed, you may have a "hidden" recycle bin which continually makes space for files which seem to have been deleted. There are probably similar programs which do the same thing.
FWIW: If you have just one big partition...I would suggest that you consider breaking that into at least 2 partitions, one for data files (music, videos, etc.) and one for the operating system and programs installed. That gives the user a better chance to recover data if something goes wrong with the boot/system partition.
As for iTunes
, I believe that it creates a My Music folder automatically when installed...which is not the same as the Windows My Music folder. It's been a few years since I used iTunes for anything, but I seem to recall that happening. There may be a more valid explanation at the iTunes forum.