I borrowed a mates game to try it, liked it and now have purchased my own copy (2 for 25 at GAME). Trouble is I installed the game with his cd key and now I can't go online with him. I can't find anywhere to change the cd key over and I'm worried that if I re-install it won't remove all the files properly and I'll be totally buggered. Is it possible to change the key or will a re-install accept a new key?

Just uninstall and reinstall the game with your CD Key and all should be fine.

I agree...uninstalling won't make a difference if you're going to reinstall again anyway. Just remember to save a copy of your custom_levels folder if you by any chance downloaded some custom maps.

Oh boy... if you unistall, make sure you have a copy of a good Registry cleaner to get rid of any remmants from the installed version... YES!!! you will have a few of those.

If by luck you don't have an issue... more power to you.