How to enable user environment debug logging in retail builds of Windows

I've found this quite useful in debugging slow logins or incorrect application of group policy.

Create the following DWORD value:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\UserEnvDebugLevel

Set:
0x00030002 - Masses of debug info
0x00010001 - Normal

It logs to:
%Systemroot%\Debug\UserMode\Userenv.log

More details here:
http://support.microsoft.com/kb/221833