Another area to check is to empty the trash! You may find under the recycler multiple trash cans that have large file space reserved even though they do not contain any files. Thes could be 100's of MB, but does not explain you 7 GB of lost space.
I too suspect that you have some hidden folders that were or are being used by viruses to hold data. Also, is your drive compressed or are some folders compressed? What is the difference between the file size and size on disk when you look at the drive properties?
I recommend that you go to disk manager and see how big the drive actually is. It is possible that it really is only using the partial drive space for the C: partition. This is especially true if the original or current OS is NT4.0, as the default partition size was only 4 GB. What is you OS?
HTH
David