function GetCD_ID(WhichDrive: string): string;
var
  VolumeName: array[0..255] of char;
  FileSystemType: array[0..255] of char;
  SerialNum: DWORD;
  MaxFilenameLength: DWORD;
  Flags: DWORD;
begin
  if (GetVolumeInformation(PChar(WhichDrive),
                           VolumeName,
                           256,
                           @SerialNum,
                           MaxFilenameLength,
                           Flags,
                           FileSystemType,
                           256)) then
  Result := (IntToHex(SerialNum shr 16, 3) +
             IntToHex((SerialNum shl 16) shr 16, 4));
end;