I believe this has something to do with the inefficient Restore function of Vista.
Take a note of how much usage you now have on your HD and write it down.
Now turn off your Restore function and then again check your HD size. I think you may be pleasantly surprised.
I suggest a reboot here, not needed but I would advise so that Vista can take a deep breath again.
Now restore your Restore function again if you so wish (recommended).
This unfortunately appears to be a bit of a problem so I am told by a reliable source.
Let me know how you went.
Thinking hurts my head that's why I live in West Australia