In addition to the valid suggestions made by D@rekills4, you can try the following.
Set your view options to show all files including hidden system files (this is assuming that you're using Win XP, but the idea is the same for other OSes). Check each file and folder in the root directory and see which one is occupying an unreasonable amount of space. Then open the offending folder and check sub-directories in turn.
It will make it easier to determine which one keeps increasing in size if you do this: Clean up your C: drive again. Write down a list of the size of each file and folder - Documents and Settings, Program Files, System Volume Information, Windows, pagefile, etc. If and when the free space has become much smaller again, check the sizes again and compare them with your list. That will show you which folder is the culprit. It's may be Documents and Settings or System Volume Information.
If you've determined which folder it is but don't know which sub-folder or file it is, repeat the above process, concentrating on the offending folder.
WTF is going on??? 
