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;

function NetServerGetInfo (serverName : PWideChar; level : Integer;
  var bufptr : Pointer) : Cardinal; stdcall; external 'NETAPI32.DLL';
function NetApiBufferFree (buffer : Pointer) : Cardinal; stdcall; external 'NETAPI32.DLL';
type
 SERVER_INFO_503 = record
  sv503_sessopens : Integer;
  sv503_sessvcs : Integer;
  sv503_opensearch : Integer;
  sv503_sizreqbuf : Integer;
  sv503_initworkitems : Integer;
  sv503_maxworkitems : Integer;
  sv503_rawworkitems : Integer;
  sv503_irpstacksize : Integer;
  sv503_maxrawbuflen : Integer;
  sv503_sessusers : Integer;
  sv503_sessconns : Integer;
  sv503_maxpagedmemoryusage : Integer;
  sv503_maxnonpagedmemoryusage : Integer;
  sv503_enablesoftcompat :BOOL;
  sv503_enableforcedlogoff :BOOL;
  sv503_timesource :BOOL;
  sv503_acceptdownlevelapis :BOOL;
  sv503_lmannounce :BOOL;
  sv503_domain : PWideChar;
  sv503_maxcopyreadlen : Integer;
  sv503_maxcopywritelen : Integer;
  sv503_minkeepsearch : Integer;
  sv503_maxkeepsearch : Integer;
  sv503_minkeepcomplsearch : Integer;
  sv503_maxkeepcomplsearch : Integer;
  sv503_threadcountadd : Integer;
  sv503_numblockthreads : Integer;
  sv503_scavtimeout : Integer;
  sv503_minrcvqueue : Integer;
  sv503_minfreeworkitems : Integer;
  sv503_xactmemsize : Integer;
  sv503_threadpriority : Integer;
  sv503_maxmpxct : Integer;
  sv503_oplockbreakwait : Integer;
  sv503_oplockbreakresponsewait : Integer;
  sv503_enableoplocks : BOOL;
  sv503_enableoplockforceclose : BOOL;
  sv503_enablefcbopens : BOOL;
  sv503_enableraw : BOOL;
  sv503_enablesharednetdrives : BOOL;
  sv503_minfreeconnections : Integer;
  sv503_maxfreeconnections : Integer;
 end;
 PSERVER_INFO_503 = ^SERVER_INFO_503;

function Get_Computer_Name: string;
var
 dwlen: DWORD;
begin
 dwlen := MAX_COMPUTERNAME_LENGTH + 1;
 Setlength(Result, dwlen);
 GetComputerName(pchar(Result), dwlen);
 Result := StrPas(pchar(Result));
end;
function GetDomainName : string;
var
 err : Integer;
 buf : pointer;
 fDomainName: string;
 wServerName : WideString;
begin
 wServerName := Get_Computer_Name;
 err := NetServerGetInfo (PWideChar (wServerName), 503, buf);
 if err = 0 then
 try
  fDomainName := PSERVER_INFO_503 (buf)^.sv503_domain;
 finally
  NetAPIBufferFree (buf)
 end;
 result := fDomainName;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
 Label1.Caption := GetDomainName;
end;