Need help, my macbook shutdown itself automaticly

Hi, i recently installed the new snow leopard sw with iwork, at the beginning everything was fine, but later started giving problems to start the machine, because i have to press so many times the button to start, but later change and now my mac is shutting down itself with out asking..... PLEASE SOME HELP, IT'D APPRECIATE

To better understand is all of a sudden you had to press the power button many times to get it to post and boot? Then now it shuts down by itself? How long after you press power does it take for it to shut down?