Replying to Hard Drive FULL?

Okay so I got the hard drive running low on memory pop up the other day... So I checked it out and saw that I had almost used all the memory up, I did a "Disk Cleanup" and I now have...

Capacity: 111 GB
Used Space: 108 GB
Free Space: 3.00 GB

I thought to my self WTF?! Then I checked folders in the Local Disk (C:) and found out the following...

WINDOWS - 3.77 GB
Program Files - 2.33 GB
Documents and Settings 24.2 GB
NVIDIA - 0 bytes
TEMP - 38.9 KB

That's only about 30 GB of my 111 GB hard drive!!!

Maybe I am making a noob error here or something, but I just don't know so help me out if you can. Thanks!