Do you have Norton system works or utilities? If you are using the Norton protected recycling bin it can be saving files that don't show up in explorer in the free space count. But are still there and their space is not available.

I would look for any programs that could be keeping deleted files saved so that they don't show up but they take up space. Or anything that could be messing up the readout of how much free space that you have.

a way to be absolutely sure would be to us a utility norton to wipe the free space.

you may want to look at your page file too to make sure that it is not taking up gobbs os space and messing up the count.

Hope this helps

David