Unable to delete DNS Reverse Lookup Zone

I have a customer running SBS 2003, and whilst looking at somethign else, I noticed a problem with DNS.

In DNS management, I have an Active Directory-Integrated Primary reverse lookup zone called 0.168.192.in-addr.arpa

When I click on this zone name, the right hand window shows the following message:-

The DNS server encountered a problem while attempting to load the zone. The zone data may not be available in Active Directory, or the zone data is corrupt."

If I right click on the zone name and select properties, I can see that the status is Expired. I've changed the Dynamic updates to "Nonsecure and secure" (it was set to None). I cannot change the type or replication because "There was a server failure".

I enabled event logging on DNS, and now get lots of 4004, 4015 and 4521 errors in the DNS Event log.

I've opened ADSIEDIT, and drilling down through ForestDNSZones, I get to CN=MicrosoftDNS and in there, there are a couple of dnsZones which show as "InProgress":-

DC=..InProgress-4EA831165436DE8E-0.168.192.in-addr.arpa
DC=..InProgress-4EA83102543690DB-0.168.192.in-addr.arpa

There were 3 of these yesterday, which I deleted, but this morning, these 2 have appeared.

If I go into Active Directory Users and Computers, and drill down to System->MicrosoftDNS, I can see 0.168.192.in-addr.arpa with a yellow exclamation on it. If I right-click and choose to delete it, I get a warning asking me to confirm deletion. I choose Yes and get an error which says:-

The object 0.168.192.in-addr.arpa (or some of the objects it contains) cannot be deleted because: The directory service encountered an unknown failure."

I'm unsure where to go with this now...