Last I knew, Nero used to check for valid serial numbers only when you tried to install downloaded upgrades. I once ran a pirated Nero 6 suite and when I downloaded an upgrade, trying to install it popped up a window saying that my serial number was on a list of known pirated numbers and the upgrade would not proceed.
If you have a valid serial number and upgrading causes this to happen to you, you'll need to send the fine folks at Nero proof of your purchase.