Computer keeps shutting down whenever I play a game or watch a video!!!!!!!?

I run my internet through Firefox (primarily) It wasn't more than a month ago I was able to play games (pogo.com) on the Firefox browser. Then it started to do this thing where the screen would go black and then a blue hp screen comes up signaling the startup of the computer! For awhile I simply went through IE when this problem would occur, but now it does the same thing. It started out with videos on youtube, moved onto games and videos and now it's started doing this anytime a flash ad comes up too. I've tried updating the javascript and the adobe flashplayer. Neither of that made a difference. Just kind of confused as to what's going on and why it all of the sudden started happening. If anyone else has this problem or knows how to fix it please help!! I work from home so having the computer shut down whenever an ad pops up makes it a real pain!!!