Seems that in Lan there is no key needed. Did some of that this past weekend.

The CD-Key is not removed while uninstalling the game. It's located in the Registry HKCU\Software\Ubi...
So don't tell us, you lost it while uninstalling the game.

If you have everything from the original game, call Ubi and get a new key - and yes, it's located on the back of your manual, so you don't need the box.