Obtendo o nome de usuário do computador

BOOL GetUserName(
  LPTSTR lpBuffer,
  LPDWORD lpnSize
);

function GetUserFromWindows: string;
var
  buffer: array[0..255] of char;
  size: dword;
begin
  size := 256;
  if GetUserName(buffer, size) then
    Result := buffer
  else
    Result := 'Desconhecido';
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  // vamos obter o nome do usuário do Windows
  ShowMessage(GetUserFromWindows());
end;

function GetUserFromWindows: string;
var
  userName : string;
  userNameLen : Dword;
begin
  userNameLen := 255;
  SetLength(userName, userNameLen) ;

  if GetUserName(PChar(UserName), UserNameLen) then
    Result := Copy(UserName, 1, UserNameLen - 1)
  else
    Result := 'Desconhecido';
end;