Shutting down...

Ok, let me get right at it. In the past maybe 2-3 months i've noticed that whenever i played some more demanding games (for myself at least) my computer shuts down after maybe 15 minutes of gameplay. Out of the 4 games i have installed right now, (NHL06, Prince of Persia 2, CounterStrike:Source, Guild Wars) only NHL does not have this problem and from the list its the least graphically demanding. Now about 7-8 months ago, i had all these games and it worked fine with it shutting down. In more descriptive detail, when ever i'd play a game after about 15min the computer just fully shutsdown. Sometimes i have to keep the plug at the back of my case off for about a minute before it allows me to turn it back up. Any ideas?

Specs:
CPU: AMD Sempron 2600+
Video Card: ATI Radeon 9200 128mb AGP
RAM: 512mb
Mobo: Asrock K7541GX
OS: XP Pro

*note everything worked fine about 6-8 months ago and all the specs were the same. I have not overclocked or really touched anything, except that i changed my old 440W PSU to a 500W PSU.

 

Some easy things to check that may be related are:

1. System temperatures. I don't use the download section here too much, but I'm certain that you will find something there. I've heard speedfan is nice...

2. CMOS battery. Lifespan on these guys are about 3-5 years. For $5, its usually worthwhile eliminating it as a suspect.

3. PSU...check that it is putting out the correct voltages. (For an ATX board, it should be 5V on the red lead and 12V on the yellow)