The links to the EA account system used to register NFS games made prior to 2008 no longer work. In the case of Carbon, the servers are still running, and apparently you can still create an online account for online play, which requires a new (unused) cd-key. Existing online accounts are still working. EA support used to be able to access the old EA account system, but I don't know if they are still able to do this, or if they were ever able to register games. The hints weren't that useful. I recall registering Most Wanted (2005) and getting a hint for Undercover 2 about a side effect related to nitrous (it reduces oversteer). You should be able to find cheat codes for Carbon by doing a web search, at least for the PC version (you didn't mention which version you have). The cheat codes are disabled when playing online, so you don't have to worry about accidentally going online with a cheat code enabled.
