games keep shuting down

Hi i have a nvida geforce9800 gt in a standard gigabite board and i am running windows 7 professional.
Whenever i run a game counter strike source or i racing the game crashes after 2 minutes of play and i have to turn the computer off at the power point because i have no response to anything at all. I have run the latest drivers for the card to no avail.
The games worked fine on vista ultimate but i decided to upgrade to windows 7 and that is when i had the problems. I was thinking that it might be ram but it seems to be a coincidence that it stuffs up right when i decide to install win 7.
If any one has any ideas i would love to hear em thank you in advance.

Do you have any errors appearing in Event Viewer (just type that in to the start menu to find it). It could be overheating, underpowered/faulty PSU or a RAM issue, but if games worked well until you installed W7 then it seems too much of a co-incidence. I'd try updating all of the major drivers for other devices, such as audio, chipset, etc...

If there is an error message in Event Viewer which mentions why the application failed, please post it here .

Thank you to all that replied. It was a faulty graphics card the fan had stopped working.