Lots of Laptops Shut Off...
I am having the same problem with my HP/Compaq nc8000. Shuts off at random times with no warning. The searches below will reveal many such cases, and usually people suggest over-heating or the batter system as being the problem. In recent months my notebook's fan has indeed been blowing more and louder, but also two months ago a battery died--strange, I thought, for a relatively new computer.
http://www.google.com/search?q=laptop+shuts+off
http://www.google.com/search?q=notebook+shuts+off