BuddyPress.org

“The site was originally installed on WordPressMU”

You can convert a WPMU install with only one site to single WP, it’s not hard to do and there are tutorials online on how to go about it. But none take BuddyPress into consideration and you’re on WP 3.0.1, so all bets are off. I would suggest experimenting on test install first and see how that works out, taking into consideration avatar and media upload paths. Main work would be on database. In the meantime, I assume that in your dashboard > Super Admin > Options > only user registration is allowed.