Without knowing more, it's always hard to say.

Generally when I'm diagnosing a crashy pc, my order of operations goes something like this:

1: Check the event logs for details of the crashes.
2: Check for viruses, check for spyware, check for other strange or unknown processes in the startup sections of the registry.
3: Check for driver updates and driver compatibility, check for windowsupdates at the same time.
4: Check components for temperatures (if there are sensors in them), make sure the system isn't running too hot.
5: Do hardware tests to rule out flaky hardware (memtest86, prime95).
6: try a reinstall of the OS and all your software.

If it's several games consistently crashing the system, and nothing else ... I'd immediately suspect cpu overheat though. What sort of heatsink/fan are you using, and how is it connected (thermal pad, thermal grease -- what type of grease). How's your cpu fan, how's your case venthilation otherwise?