Games freeze

what caused it to start freezing, and what were the most recent changes you made before it started?

If it happened to me, I'd try to scan the PC with both updated AV and Spybot, then verify the game's cache.
A freeze like that could also be CPU/GPU throttling from power starvation, so I'd suspect the PSU or the motherboard.
It could also be a defective RAM module, so I'd also try a Memtest run.

I remember something like this happening to my old,old PC in 2008 or 2009 ... it turned out to be a virus which literally had blown my windows to bits :D

only games freeze? Can you alt ctrl del when it freezes and go back to the desktop?
Maybe the vga can't output video and that causes the freezes.

I had to replace my 660ti e month ago, and it was playing metro redux just a moment before dying (all i did was cable management outside the computer). The vga is the first thing i'd point out in a gaming rig