Why is my computer crashing?

[touch]

My computer frequently crashes when i try to load games. Games i play most are flight simulator X, CoD2 and CoD4. As the game is opening, the screen turns off and shows "entering power save mode".
It is not a problem with the monitor, i have tried 3 different monitors through both VGA and DVI connectors on the graphics card. Occasionally, when the computer crashes, i can hear a little bit of the sound looping over and over so i'm quite sure that the computer is crashing and not just changing to a resolution that my monitors wont display.

I'v always blamed this problem on windows vista, it's a dell pc that came pre-installed with vista home. I thought that it could have been a bad install or something so i'v just wiped it and put an old copy of xp that i had on it but i still have the same problem (in fact, maybe slightly more frequent crashes).

Spec is:
core2quad Q8200
3GB RAM
Radeon HD 3650

 

[Orcvader]

Could be just me or it's because the 3650 can't cope with it all...

 

[touch]

Could be just me or it's because the 3650 can't cope with it all...

I know it's not the best graphics card, but it should be able to run these games.
It does not crash every time i try to open them, and the times that they do load they will run fine (good fps, not looking like it's struggling at all) and generally wont crash half way through a game - only when it's first loaded.