First thing that comes to my head is, is it my power supply? Time to replace it?
And I also noticed this only happens when I play games, so using my graphics card/accelerator more.
When I'm just on the forum etc it doesn't happen as much, but games, it'll work for 20 minutes and then just switch off.
And now it's gotten to the point where I have to try about 5-6 times for it to actually turn on. I hear the "beep" for the hardware check but after that it just doesn't go anywhere, and when it starts up, my screen goes blue and I get this message
"A problem has been detected and windows has been shut down to prevent damage to your computer.
The problem seems to be caused by the following file: avt2dvag
The device driver got stuck in an infinate loop. This usually indicates problem with the device itself or with the device driver programming the hardware incorrectly."
I'm guessing it's either my power supply or graphics card..

Right now it's working after like 6 tries!