If you are positive you have no viruses I would suggest making sure all of your drivers are up to date (for your gfx card etc.) This might be the problem as you said 3 out of the 4 times it has happened was during playing games, and the fourth was just after playing a game.
If you are still at a loss after this you could try doing a system restore to return to a time when you computer was functioning correctly. Before doing the system restore I would suggest backing up your more important files to an external hard drive - just to be safe.
If this still doesn't work if you still have a copy of your Win 7 disk you can always reinstall it. Although this is a long winded process and involves backing up ALL of your data before hand.
However before you consider this option (presumably because all other options have failed) i would seriously suggest looking around other, more specialised forums for an answer.
One way to test whether is due to games/your gfx card is to simply not play games for a while and see whether your PC locks up, and if it does, what were you doing exactly before it did. This might help you narrow the problem down somewhat.
Hope this helps
