WinDirStat is an opensource tool to identify howmuch of disk space you are using on your wondow box. It shows disk, file and directory sizes in a treelist as well as graphically in a treemap, much like KDirStat or SequoiaView.
Once i started using it, i feel like life would have been very hard without it.
Why don't you try it yourself.
You can download the tool from http://windirstat.info/