A copy of the original hood is located in C:\Program Files\EA GAMES\The Sims 2\TSData\Res\UserData\Neighborhoods.
You got to copy and paste the N001 from there to the Neighbourhoods folder.
The NeighborhoodManager.package is regenerated if you ever delete it, I believe. The neighbourhood folders aren't - fortunately, since that's my idea to limit my selection of hoods to play
Or more shiny ... grab the clean neighbourhood templates, lovingly townie-free, found somewhere in Peasantry.
EDIT: Geez, 5 minutes lag. I'm so lame.