partition full, but only can only see 6.63 gigs out of 20.

My 'C' drive (also my system drive) seems to be full. The computer is telling me that I only have 14 megs of space left on my drive. But when I go into it and select all, I am only showing 6.63 gigs if stuff on my drive. Ive deleted unnecessary files and folders and programs and cleaned out my drive as much as possible, but its still full.

I just opened up about a gig and a half about a week ago, but now I have somehow managed to fill all that up and only have about 14 megs remaining, even though I haven't added any programs or stored anything into my 'c' drive since then, that I know of at least.
The thing that I don't get is that I know I don't have 20 gigs of data on my 'c' drive. I have only stored system files and folders and basic programs into that drive, around 5 or 6 gigs of stuff. Where the other 15 gigs came from I have no idea.

I've checked with my 'computer management' tool and it tells me that I have almost 20 gigs of stuff. When I highlight my 'c' drive on the 'my computer' screen and select the 'properties' link, it tells me that I have almost twenty gigs. But when I open the partition and select everything that I see, it only shows 6.63 gigs. Could I have 14 gigs of hidden files and programs on my system drive that I don't know about. And if so, how do I access them to see what they are.

I'm running with windows XP pro and have an 80 gig hard drive with an athlon xp 2200 and 512 megs of ram if that helps.

Another thing is that about 6 months ago, I tried to delete my entire hard drive and reinstall the OS again for a clean start. I deleted all other partitions except my 'c' drive cause it was the system drive and it wouldn't let me, so I just tried to reinstall the OS on top of the existing system partition. It worked but showed that I had about 14 gigs of data on that partition already, even though I couldn't see or access any of it. I tried everything but could find the 'invisible' data. So like a lazy bum, I just let it go and now my partition is full and I can't delete any more programs from that partition cause I need everything I have left. I've tried moving some files, but that didn't free up much space. I've also tried to inactivate my system restore function and that didn't help either.

I want my 15 gigs back. Any ideas on how to do this would be greatly appreciated. Thank U.