Missing Hard Drive Space?

Fresh Windows 7 Ultimate SP1 Installed and updated with a few programs installed and some audio files.

Just realizing that I am running out of space on my hard drive fairly quickly.

Under My Computer(C Drive) it lists 28GB free of 110GB total.

I have done the math(added up the C drive files, including the hidden files) and I should have around 54.4GB free, not 28GB.

I know that pagefiles take up 1.5*RAM installed, I only have 2Gb RAM = 3.5GB paging file. And possibly a hibernate file of the same size for a total of 7GB extra taken up. But the math still does not add up.

I have confirmed this by running windirstat, which says I am using 48.6GB on my C Drive(110 - 48.6 - 7 = 54.4)

The Math:
+110 (The Hard Drive)
- 3.5 (Paging file)
- 3.5 (Hibernation file)
- 48.6 (Everything visible[and hidden] on C Drive)
----------
54.4 GB

Which is 26.5GB more that what I have listed(27.9GB)

I have also not made a backup yet, so as far as I can tell that is not taking up space, and the C drive is 0% fragmented.

Any help as to what would be taking up this space(26.5GB) would be greatly appreciated.

Thanks for the quick replies. I do have avast installed, I'll check that a bit later and get back.
Been looking for a good excuse to get rid of avast for a while now, this may be it.

Ahh perfect :) That freed 35GB!(10GB over the maximum allowed in the restore point settings) Thank you everyone for your help, I used all of the links.

I knocked it down to 2GB in the configuration settings, seems to be enough, one restore point is about 120MB for me.
Thanks again