If you happen to run a large Windows environment you might be familiar with the in place upgrade method of upgrading your domain to Active Directory. If you run a large Windows environment that spans several sites over a variety of links then you will know that an in place upgrade can be a pain.
The main problem with an in place upgrade stems from the fact that client machines will always prefer to talk to the Active Directory server instead of the Windows NT4 Backup Domain Controller. This means you can end up in a situation where a remote sites clients are traversing a poor link to authenticate against the Active Directory server and ignoring the local NT4 Domain Controller.
To work around this issue Microsoft provide a registry hack called the Windows NT4 Emulation key. If a DWORD key called NT4Emulator is created in HKLM/System/CurrentControlSet/Netlogon/Parameters and given the value of 1 is created then the server will 'pretend' to be a Windows NT4 server thus the client machines do not see any Active Directory domain controllers on the network and so will be quite happy to authenticate locally.
I'll cover this key and some of it's drawbacks in some later articles.