Laptop shutting down. My laptop shuts down while watching internet videos. It used to be just on

Question

My laptop shuts down while watching internet videos. It used to be just on mozilla so I would always watch on IE. Now it occurs on IE also. Now I can watch DVD's for hours and it never shuts down. Why would this occur just by watching internet videos? It now happens within seconds of the video starting so how can this be an overheating problem? It sounds more like a software problem, but then again if I knew the correct answer I wouldn't be here!Thanks for the help.