Did you look at hidden files? I always set Windows to show me everything. It is quite possible that the disk has lots of hidden stuff.Originally Posted by another view
A less likely situation is that you somehow changed the minimum size that each file takes up. File space is allocated in blocks. If you block size is 10 M and you file is only 1 M, it still takes up 10 M of space (bad numbers but the correct idea). This is the same for all computers. This can happen if you used some "tune-up" program or installed some weird software.
Also do a spyware/adware scan. These malicious programs can eat up space as well as slow your computer down.