Seing more space used than the added sizes of the files is normal, it comes from the fact the NTFS system uses 4 KB clusters by default. A 1 KB file will use 4KB on the disk (in fact even a 1 byte file will use 4KB), a 1601 KB file will use 1604 KB on the disk, and so on. If you have lots of small files, that wasted space adds and can become quite big. Though it would use less space, you don't want the clusters to be smaller because 4KB is also the size used by windows to handle the ram and same cluster size = faster transfers from hdd to ram. Bigger clusters also allow bigger transfer rates and smaller file allocation table. 4KB is a nice compromise.
Now you still have a problem with that unrecovered space. Did you empty the bin ? (dumb question, I know) Is there some kind of file monitoring program running in background ? Check the size of the swap file is not too big : right-click on my computer => properties, advanced, performances, advanced, change under virtual memory. Max setting should not exceed 2GB. Run the disk cleanup utility.
I don't think it could be linked to system restore, since that usually only creates restore points when a new program is installed, but you could try disabling system restore for drive G: (right-click on my computer => properties, system restore, choose G: and click on settings).
Next option would be to post a hijackthis report in the
dedicated thread and see if there's not some sort of malware on your computer (read the "important" sticky first in the hijackthis thread)