Sony Vaio shuts down automatically

 

I have Sony Vaio laptop that has Windows XP Media Center Edition.  Recently
through Automatic updates operating system was upgraded to SP3.  While
upgrade I did not get any errors.  Since last few days, after about a couple
of hours after I start my computer shutdwon window containing stand_by,
turn_off, etc., gets displayed.  Even if you try to click any option computer
powerd off without performing proper shut down.  I would like to know if this
problem is due to Windows or if it is SONY Vaio problem.  Please help me.
Thanks in advance.