Dawn of war dark crusade troubles!

so me and a buddy got a craving for dawn of war 1 this evening and really would like to play but i dont have a valid cd key, so i got ahold of another, in a completly leagle way:angel: but now i have no idea how to change the key to that, does anyone know where abouts in the program files or in any other way i could insert this, and just for the record reinstallation is completely impossible and i need a way to do it without removing the files. thank you!