Help please

If any one can help it would be greatly appreciated:

I'm not sure if i have a virus or not but when i try to watch videos on my MacBook pro the video freezes but the sound keeps playing and then shortly after the sound stops and the computer restarts without warning. While on the white apple screen a window pops up telling me that there was a problem when the last time the computer was shut down and to hold the power button until it shuts off and then press the power button to turn it back on. It says this in several different languages. When it turns back on it works fine unless I try to watch a video again. Does anyone know what the problem may be and how I can fix it?