Computer reboots when playing games like Saints Row, Dawn of War 2 ...

[Earth]

Hello everyone
For some time now, in some recent games (Saints Row, Dawn of War 2 ,...), my PC freezes and then spontaneously reboot ...
- I first thought about the RAM, but the test passes by on memtest86 + did not reveal anything?
- I then thought that the graphics card might be overheating but I removed the cover of the fan and heatsink: The heatsink was a bit dusty but nothing more. I cleaned and vacuumed the cotton swab blunder very well ...
- I even spent time with 3dmark 2006, history of overloading the system a bit for the game (I guess) but no reboot during the test.
- There are no errors or warnings in Event Viewer.

So cant think what can be the problem here. Thank you in advance for your lights bestowed upon me.

[Amil]

Obviously, your power supply might be at fault then.
Possibly, you can calculate the power of the PSU that you need: http://www.corsair.com/psufinder/default.aspx

[ApPy10]

Try this if will have already accomplished:

By default, Windows XP is configured to restart on error. To stop the restart, a simple enough sometimes, but it is not effective in all cases:

1) Go to Start, then Control Panel then Performance and Maintenance and then System (or by right clicking on "My Computer" and selecting properties)
2) Click the Advanced tab
3) Click Settings in Startup and Recovery
4) Uncheck "Automatically restart"

[Amol21]

Try to disable your antivirus (you never know). In advance ofcourse disable your Internet connection.