[RusMaXXX]

Код:

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('VolSer = %8.8X', [VolumeSerialNumber])
end;

применять так: ShowMessage(GetHardDiskSerial('c'));