Windows Vista update trouble

My windows update is freezing my computer.  It has failed to update Windows Vista Service Pak 2 for X 64 based system.  I will be working on the computer and it just freezes and says updating do not turn off computer.  This is on for days.  I have tried to do many things but I can figure this out..  The only way to get rid of it is to shut off the computer.  Sometimes the computer won't come back on.  This is so frustrating.  Please help!