As tman said, it would be complicated. You would need something like GoBack on the hard drive side, and something to grab and save memory to disk every time GoBack sets a major savepoint. And while it happens, the machine would have to freeze, or have double the memory to just copy RAM real quick to then go to disk. And if something goes wrong in matching up the disk and memory, well minor things like programs crashing could occur, or if something critical is affected, Windows would blue screen.