Автор: http://www.sources.ru

function GetNTDomainName: string;
var
  hReg: TRegistry;
begin
  hReg := TRegistry.Create;
  hReg.RootKey := HKEY_LOCAL_MACHINE;
  hReg.OpenKey('SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon', false);
  Result := hReg.ReadString('DefaultDomainName');
  hReg.CloseKey;
  hReg.Destroy;
end;