Maybe try something like the free version of TreeSize:

http://www.jam-software.com/freeware/index.shtml

It scans the drive and lists the entire drive's directory structure, showing the size of everything.

Not sure if that will help here, but in general it's a great tool for figuring out what the hell is sucking disk space.

I assume you're logged on with a profile that has permissions to everything on your system? No pesky kids removing Admin rights from folders to try to hide their porn?