I notices that I was low on disk space in one of my machines with a small disk volume on the system partition. So with the help of my handy HDGraph I was able to locate the area of my free disk space consumption.
It seems that the Windows Error Reporting Service was dumping logs on programs or applications crashes into this location. I normally do not review or send these logs out so I deleted them and gained back 10GB of space.
This is something you want to check in on from time to time.
More Info: http://technet.microsoft.com/en-us/library/cc754364.aspx