XP SP3 Breaks Office Peer-to-Peer Network

All machines running XP Pro. Those updated to SP3 are no longer accessible on ptp network. Not using simple file sharing. We’re part of a corporate network but only connect occasionally through VPN so we’re joined to a domain but operate apart from it. Not defined as a workgroup.

Working with Microsoft on this but haven’t had time to troubleshoot it yet.

Any ideas?