Download
Space Monger which will examine your disk and show you coloured blocks that correspond to the files on the disk. You may find that you've got a few huge files on there taking up all your space.
I found 11 hidden files with various filenames, all letters of the alphabet and each one 1,572,072KB in size (16.4GB total!) hiding in C:\Windows, but I can't do anything to delete them. Loads of searching has yielded nothing, but my drive is 120GB, so I'm not worried, but it would be serious for you perhaps. I think a format & reinstall would fix it, but that's quite drastic.