full screen problem

ok i download a game and when i start it its in full screen mode but i can only see the top left of the screen the other 3/4 of the screen is black and when i move my mouse onto the black part my mouse gets covered to also when i lower my computers resolution the same thing happens but not as bad just a little of the right side gets covered up. i had about 5 games that i got online that do that and all are from different sites and i had them on before and worked fine
my computers a xp
can any one help