HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon
On Windows XP, the name appears in the DefaultUserName value:
But on my Windows 7, the DefaultUserName value is absent:
This has been noticed before. That person says it works sometimes and fails other times.
I don't know why it's absent. Even creating a second user, logging out, and logging back in with the other name did not make it appear.