Have an engineer whose computer died, when attempting to install their software on a new computer I get an error stating that the license key is invalid. Went online and setup an account and attempted to register the product there, and receive an error as well (The serial number you entered is not valid. Try again. (1)). This is a single seat software license for Autodesk Navisworks Simulate 2011.
I have confirmed the license key was entered correctly (I have the original packaging / label).
You can manually activate software. Read the info noted in this blog post: