Where has my hard drive space gone?

I have a Dell XPS M1530 laptop with a 320Gb HDD. It came partitioned as a main C drive with a recovery D drive.

It had Vista on it to start with, but I have just done a what I thought was a clean install of Windows 7. Windows is showing the size of the C drive as 285Gb of which it says 192Gb is free, so 93Gb in use.

Fair enough you might say. However, when I add up the size of all the folders on the drive, including hidden ones, I only come to roughly 48Gb, which means there is a discrepancy of 44Gb. Some of that will be the swap file, but still it seems like a huge difference.

Any thoughts?