The missing GB is usually the total of the page-file, hibernation file, the master files and the system restore points.
To free up your GB
Disable Hibernation (you'd free up as much space as you have RAM)
Disable System Restore (Usually freeing 1-4 GB)
Disable pagefile (which is not reccomended, but can free uo from 300MB-4GB)
The Master File can be changed but you'd need a utility to do it (usually less than 100MB, i think :cheesy: )
If you don't know how to disable them, post again.