WinDirStat is a disk usage statistics viewer and cleanup tool for Microsoft Windows (all current variants).
This program is super-useful for finding out what exactly is taking so much space on your hard drive. It can sometimes take a long time to scan your computer, but it works really well regardless.
It also lets you visually see your hard drive. Here's mine:
Basically look for all the big stuff
My movies folder is shown highlighted in the upper right. The big blue below that is a DVD image backup for Oblivion (4.3 gigs.) The lime green ones are back-ups for Counter-Strike (I'm thinking about getting rid of them.) The light blue at the bottom right is the page file, and I didn't check what the other things were.
It can also display free space, too, but about half my hard drive is empty so it would be a waste. It also needs to redraw every time you resize a window, but it's super fast and isn't much of a problem.
Check it out, tell us what you think.