what about the Add/Remove programs list and sort by file size?
There is an advanced xp powertoy thing that is basically the same thing but it doesn't show size, just everything installed. I don't think you have 12gb of hidden programs though... That is odd. Could just be really fragged. Have you tried partitioning and then deleting temporary files using the "clean up system harddrive space" button on the harddrives property panel?