function GetHardDiskSerial(const DriveLetter: Char): string; 
var 
  NotUsed:     DWORD; 
  VolumeFlags: DWORD; 
  VolumeInfo:  array[0..MAX_PATH] of Char; 
  VolumeSerialNumber: DWORD; 
begin 
  GetVolumeInformation(PChar(DriveLetter + ':\'), 
    nil, SizeOf(VolumeInfo), @VolumeSerialNumber, NotUsed, 
    VolumeFlags, nil, 0); 
  Result := Format('Label = %s   VolSer = %8.8X', 
    [VolumeInfo, VolumeSerialNumber]) 
end; 


procedure TForm1.Button1Click(Sender: TObject); 
begin 
  ShowMessage(GetHardDiskSerial('c')); 
end;