HDD space disappearing

For some reason my HDD space is just going

I have 931.5GB drive

SpaceMonger says 372.5GB free and 407.3GB Used which is 779.8GB

Defraggler has lost me 151.7GB on my drive

Also this is not my windows drive because I have seen other threads, there is no hidden folders its just gone

Is System Restore set to monitoring your drive??

Richard S.

thanks redhawk got back my space, was very sure that I had turned it off.