Want to remove old windows

Hello, hi i purchased my computer online about six months or so ago ,never thought anything of it till i got a message saying that windows copy you are using maybe counterfeit,you could imagine what was goin through my head ,anyway everytime i would try to load any game to play on the pc it wouldnt load it .i have a full copy of window xp sp2 to put in to this computer , what do i do next as i want this copy of windows removed and my new copy put in . cheers any help would be great thanx ps it was brought so i could use it for gaming unaware of the things i couldnt actually do because this computer was one of 30 or so computers loaded with the same windows cheers hone26