Free space disappearing for no reason
About three years ago I purchased a Dell XPS 8100 computer with a 1000 gigabyte hard drive, Windows 7 Home Premium with an Intel Core i7 CPU. Until recently, this computer has performed wonderfully and I've always had approximately 850 gigabytes of free space.
A month ago, in one day, the free space decreased by about 90 gigabytes. Yesterday, in approximately 12 hours, it decreased by another 110 gigabytes. I've not downloaded anything out of the ordinary during this time, as I mainly use this computer for emailing and surfing. The Dell site checks indicate all hardware is okay (although at one point it said "Unexpected Error Occurred"), and checks by Microsoft Security Essentials, Malwarebytes, SpywareBlaster and Spybot indicate all is well.
I would definitely appreciate suggestions.