Get the current Username

function GetCurrentUserName: string;
const
  cnMaxUserNameLen = 254;
var
  sUserName: string;
  dwUserNameLen: DWORD;
begin
  dwUserNameLen := cnMaxUserNameLen - 1;
  SetLength(sUserName, cnMaxUserNameLen);
  GetUserName(PChar(sUserName), dwUserNameLen);
  SetLength(sUserName, dwUserNameLen);
  Result := sUserName;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(GetCurrentUserName);
end;