@Zazazu: I'm pretty sure that all my backups are in my external hard drive, so that can't be it.
I even redownloaded the files and placed them into the EA Games/The Sims 2/TSData/Res/UserData/Neighborhoods/N001 folder over the old ones.
@B: I'll give that method a go. Hopefully that's the trick.
EDIT: Lo and behold, taking out the My Documents/EA Games/The Sims 2 folder did the trick. I created a new neighborhood without incident.
Now, I hope that I won't have to do this each time I want to create a new hood...