procedure TForm1.Button1Click(Sender: TObject);
var
  SerialNum : dword;
  a, b : dword;
  Buffer  : array [0..255] of char;
begin
  if GetVolumeInformation('c:\', Buffer, SizeOf(Buffer),
    @SerialNum, a, b, nil, 0) then
      Label1.Caption := IntToStr(SerialNum);
end;