Try treesize a freeware program that can show folder size incliuding all sub folders. it will add a right click option called treesize to explorer.
Right click on the drive letter and click treesize to see which folders are using up the space
I use this program often when doing spring cleaning on my PC
There is a screenshot of the application below as well
http://www.jam-software.com/freeware/index.shtml