I had this with my drive, it turned out to be a shit load of music and other stuff (downloads etc) in Computer>Local disk (C:)>Windows old>users> (user name)
This was with an upgrade though, So probably doesn't apply.
On another note:
I've just got myself a nice new monitor LG 32LD450 and got a DVI to HDMI cable. I'm running 1920x1080 Resolution and all the writing is choppy, and Watching HD films is definitely NOT HD. Basically it's a really crap picture. Any insight? Drivers, cable, etc?