windirstat is a great utility to work out how much disk space each folder/file is using. When you get down to the file level it will show you in a graphical format as the larger the file, the larger the colour block on display. Using this utility it was very easy to find out where the 12gb of disk space had suddenly gone on a server – it quickly showed me that a particular user had a large docs & settings and within that was the desktop and on the desktop were 2 6gb .tmp files!