Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Try this from the Terminal:

sudo mv /var/db/netinfo/local.nidb /var/db/netinfo/local.nidb.old

If you want to do it from Single User mode:

mv /var/db/netinfo/local.nidb /var/db/netinfo/local.nidb.old

Then restart. Either one will make it re-run the registration thing.
 
If you don't want to inactivate your old NetInfo DB, you could try this:

Just delete this:
/private/var/db/.AppleSetupDone

If you delete this file, OS X will "think" you haven't done the setup, and will launch it again for you.
This could come in handy if you don't want to remove your account (like a system admin-account), but want to force a new user to create his/her own account, after a first boot.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.