My Win 7 was shutting down after being on line for 10-15 minutes. I tried everything....restore....update drivers....etc. Finally I took the side panel off and you would not believe how dirty the inside was. The video card fan was totally clogged up. Ditto for the case fan. After a good cleaning everything is working fine!
My advise....CLEAN THE INSIDE at least every two months, especially if you have inside pets. This will probably stop a lot of your problems.