Odd gaming issue

I have been trying to play a couple of different games (Diablo II and AOE 3) lately with the same problem. When I start the games, they come up fine and then I go to begin a campaign or continue from my last save point (etc) which generally also starts fine, then it happens...

My computer dumps me back to the desktop.

The game is still running, but I have to go click on it to bring it back. Then, anywhere from mere moments to minutes, it happens again.

I cannot figure out WHAT has happened within the system itself to cause this issue. I recently defragged my C: drive thoroughly (something it needed in a bad way) but other than that I've not done anything significant. This machine isn't even connected to the internet, it's just there for gaming and other entertainment or work (MS Office type) purposes.

If anyone has any ideas, please post them. Is there a possible corrupted system file? Another thing that happens now when I boot is I have an error regarding the Kill Process function (says it "failed" and asks me to report it to MS which I can't do on that PC since it is not connected to the 'Net). Those are the only two "symptoms" I can offer.

I'm not new to technology by any means, but this one really threw me.

 

A second note here:

1. I have updated to the latest video driver for my system (so that's not the issue).

2. It isn't just games. What seems to be happening is the system loses sight of whatever the active window is. Even if I am working on something (non-game related) the active window suddenly becomings inactive and I have to move my mouse over and reclick it to make it active so I can continue my work.

I think that is the core issue, the system is losing sight of the active window. If there is a system file that controls this that I can replace, please let me know. I'd rather not have to re-install my whole darn system.