I have installed & registed EIC, I then downloaded the latest patch and tried to install. However, the installer required me to put my CD key in again, which I did, but it came up with a popup reading 'There was a problem with product activation:
Serial Number already registered'
Which it is, I used it to register the actual game.
Any idea what is wrong?
This problem has happened before. Try under tech Support. There is something that may help there under Sticky: Activation problem? Basically you have a problem with the activation company rather than the game.