A. If you receive an 'Access Denied' message when attempting to promote a BDC to the PDC it may be due to the fact the PDC has Service Pack 4 installed.
This is because Service Pack 4 upgraded the security mechanism used so you will either have to perform the promotion from a Service Pack 4 domain controller or upgrade the BDC in question to SP4.
Another reason for this error is trying to get a renamed and upgraded (3.51 to NT4) server to sync with the domain. The accounts database may have become out of date and thus couldn't be synchronised. NETLOGON may not even be startable.
The way round is to do a "connect as" from the PDC to the rogue BDC using an admin ID known to be good by the BDC before it was upgraded. Once the "connect as" (say to Cc) was accepted, the BDC would then accept the synchronise request from the PDC's Server Manager, restarting NETLOGON in the process.