Vista - system does not support full screen mode? please help!

i mainly bought my computer so i could play world of warcraft on it where
ever i am in my house. now i cant play any games that require full screen
mode, becuase i get an error message that says, "system does not support full
screen mode"
how do i fix this? i ran into a website that had a hotfix but it had a
warning so i didnt download it but thought instead i would ask around.
thank you very much for your help, and merry christmas!