CD KEY wont authorize

I have a valid CD Key which came with my Doom 3 game - I can play offline games perfectly. However, when I join a Online Multiplayer Server my installation always seems to freeze upon reaching the "Waiting for Authorisation Screen" and if it isnt this I get a CD Key box popping up telling me to input my CD Key. What is happening - my CD Key is fine - I have double and triple checked and re-entered it about a million times. Even in the multiplayer options box and even there it says it is Valid. What is Happening - Why wont it work???