Constant Restart

Hi all,

Sorry was confused as to where to post the below issue.

my configuarations

core 2duo e7500 2.93 ghz
Kingston 2gb and dynet 4 gb ram
gigabyte g41 m combo
500 and 320 hdd

I had installed win 8.1 which was working good until I started getting frequent restarts this happened once twice then suddenly my pc use to restart as I connect any mobile phone it wont when I connect pen drive.

now it just restarts as soon as the welcome screen comes
yesterday I installed win 7 win 8 installation goes very well bt as the installation finishes it restarts

totally confuse is it the hardware issue as if it was hardware issue it wont happen even while I was installing the os

if it is a software problem it should not arise in other os also I have installed the same os in other computer

can u pls help as to what possible I can do in order to get that replaced or any solutions

Many Thanks.