Here is a very useful utility called JDiskReport which I find extremely useful. If you're trying to find out exactly where all your valuable disk space has disappeared this is just what you need. When installed, if you right click on any directory it will give you the option of displaying the size of the various subdirectories underneath it in the form of a pie-chart. Clicking on any of the segments representing a directory in the chart produces a new chart showing the size distribution of that particular directory. I used to spend ages doing this kind of thing manually.

The program is free and has no advertising or other nasties included as far as I'm aware. It can be downloaded here,
http://www.jgoodies.com/freeware/jdiskreport/index.html