My Biggest Problem with my LapTop

[Golden Pen]

I have Big Problem in my LapTop (HP Compaq NX9010)
it’s Shutdown Suddenly

When i use it sometimes I can't move the mouse and after 2 sec It's shutdown suddenly
and what i see it it's a black Screen

I want to know the Causes of this problem

2 Causes in my mind:
1- Is that happening by the viruses ( I use KAV 2010 ).
2- Is That Happening because i work for a long time ( i work on it for 5 - 8 hours for a day )

Why that is happening for me?!