Wow, some people try to make this crazy hard. Here it is from the CLI. Or as a .reg file:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
"AutoAdminLogon"=dword:00000001
"DefaultUserName"="user"
"DefaultPassword"="password"
This seems to be available from at least Windows 8.1 on.