Projector problems with my lap top

Question

I have vista on my lap top. Everytime i plug in the projector to it, the screen flashes on and off and so does the image on the wall from the projector. It will sort it self out and then within a minute or so ir starts again. I have changed down the resolution on the lap top and it still happens.I am unable to use the projector with it. It works fine with my mates laptop.Mine is a compaq presario F500.Any ideas please ?