Black screen / resolution change when watching video's

first off i do not know if this is in the right section... i apologise if it is not.

My problem being when i watch video's on say megavideo every now and then, or sometimes continously the screen will go black, then the image will return, sometimes the image will be normal, sometimes it will be larger then my screen and i have to exit full screen mode to get the image to return to normal size. When this happens and the screen goes black, windows makes a sound as if its found new hardware... that kind of sound. I thought it was just something i had on vista cuasing it but ive installed windows 7 since and the problem continues, also it happens when i play a game, and when the screen comes back on the game has gone to window mode.

Im asking for help now as it has just done it again whilst watching a video but this time the sound continued but the screen never came back on, and i had to restart my computer.

Thanks