Hello and welcome to Techspot.
Open your documents and settings folder, at the top of the window click on tools, and then folder options.
Click on the view tab, and check the box that says show hidden files and folders, and untick the box that says hide protected operating system files (recommended). Click yes, followed by apply/ok.
Can you now see the full ammount of used gigabytes?
If you can then, that`s where your missing 20 gig is.
When you are through looking, just reverse the above instructions.
Regards Howard :wave: :wave: