Windirstat

I don't give out my recommendations lightly.  If i haven't used something, i'm not going to tell you that i like it.  I might say to give it a try, but i won't tell you that it's the best thing ever without knowing.

I've used Windirstat (Windows Directory Statistics) for several years now.  It is the best tool i've found for tracking down why your hard drive is full.  You can get it at http://windirstat.info/.  The actual code is hosted on SourceForge.net, which has always met my needs. 

Download windirstat and run it.  Let it analyze your C: drive to see what you get.  Besides creating an awesome visual representation of your hard drive files (btw, go to options and turn on viewing free space), it'll list the folders on your hard drive that are consuming the most space. Ever run out of space on your hard drive and wondered what you could delete that would have the biggest impact?  That's what Windirstat can do.  It'll even look in your recycle bin to see if anything in there is taking up significant room.