I need to clean up C:

Disk drive is like, under 10 mb of space and it's messing up my downloads for games. I can download a 737 MB install executable and then when it's finished all it gets is like, 37 megabytes. I have a G: and D: drive with plenty of space on it but my C: drive is out and I have trouble cleaning it because I removed every thing off of it already. I assume it's a virus of some sort and Ive tried disposing of it, but it hasn't worked.
Disk Cleanup doesn't do any good.
IS there any way I can clean up my disk drive with out having to wipe it out or spend money?