ZT Computer keeps Shutting Down

Hello All, I have a new dilemma that is truly bizarre & frustrating.
My desktop PC will be on for about 5 minutes & shut down & then the blue screen pops up
Saying it had to shut down to prevent damage. The computer has been in the shop twice in
A month & the guy who fixes it says it works fine at the shop without shutting down. We got it back
today & it shuts down after a few minutes & blue screen again. What can the problem be?
Also what's been happening although maybe unrelated, is we have problems opening some
Pages on our cell phones. It appears to be a reception problem(?)
Any fresh insights much appreciated.

Marina .

P.S. I'm using Windows 7 Home Premium

Never mind, I found the answer myself. I used system restore & that did the trick.

Thank you for letting us know.