Help--restart problem

My computer restarts when i am trying to turn it off. Just bought it last week, could it be motherboard problem? It doesnt restart when it is on, only happens when i want to turn it off. And it happens randomly, i.e. sometimes it turns off, but sometimes it restarts about 5-30 seconds after it is turned off. And I didnt press any button. Does anyone have any idea? Sorry cant type chinese cuz havent installed chinese input yet.