Windows Vista: Unknown disk space allocation

Hi!

I've bought the pc, whose I'm writing, 3 weeks ago.
I found that the unit C:\ has 70.7 GB of free space, but I haven't put in my
HD this amount of memory (73.3GB).
If I go to "My computer", then in directory C:\ and I press 'Select All and
then Properties', it appear that I've used "only" 50.3GB.
I ask you what is the source, of the "lose" of 20.4GB

And I find this so uncommon, because I think that I haven't putted 50 GB
(including the OS) in my main HD.

Thank you in advance for your replies!

Netlopa