Have you cleaned the cpu fan? The
computer has a cpu fan that can get dirty over time. It's job is to keep the cpu cool. When the cpu overheats, the computer senses it and shuts it down immediately, to prevent permanent damage to the cpu. You can clean it by blowing compressed air into it, and should do that every few months, as routine maintenance. This is the most common thing that shuts a
laptop down, other problems involve repair, so try this first. I bough a $7 three inch fan at Walmart to help cool the laptop, and when I run it, I never have it shut down on me, but once in awhile it will without the fan, especially when I play dvds or videos, which really work th' heck out of the cpu. There are also commercial laptop cooling devices, but my little fan works just fine.
The other thing that could be happening is some problem with windows, which is something I couldn't begin to guess. Hope it's just a bit too much heat.