|System Restore takes up quite a lot of space for it's accumulated restore points, but that would only account for 15% of the total capacity - so around 15GB at most on your disk.|
Another space-eater are some security utilities which wipe the free space on the hard drive to prevent recovery of deleted files.
Some of them write a whole series of large, random-data 'padding' files to fill the free space, depending on the erase method you choose, and they stay there until manually deleted. I found 30GB of such files in my user account folder after running one of those utilities.
Third possibility is malware. Some malware is crafted to keep filling up the free space with junk files. You'll need to run several different on-demand malware scanners to check for that.