what is ntoskrnl.exe?it caused a bsod!

firstly i found it is my nivida (n2_dis.sys or something)then this time it says its the ntoskrnl.exe..what is it?how do i update it?i hope updating this would solve it as i got 3 bsod today again!!!
 

My Computer

System One

  • CPU
    Intel(R) Core(TM)2 Quad CPU Q8400 @ 2.66GHz
    Motherboard
    Elitegroup P43T-AD3
    Memory
    4.0GB Ram DDR3
    Graphics Card(s)
    GeForce 9600 GT
    Sound Card
    Realtek High Definition Audio
    Monitor(s) Displays
    Lenovo 1440 X 900 60Hz
    Screen Resolution
    1440 X 900
    Hard Drives
    1TB
Top