Most FAQ recommend use GetVolumeInformation to get "harddisk serial number". But it is volume serial number, not harddisk s/n. It is assigned and changed on formatting partition. Some companies
use cloning tools to install software to all new computers by copying
from single harddisk to all another. Of course, all volume infromation on these computers are identical.

I recommend other way: get real harddisk serial number.
Unfortunally it works for IDE harddisk only.

// Get first IDE harddisk serial number
function GetIdeSerialNumber : SerialNumber;
const IDENTIFY_BUFFER_SIZE = 512;
type
TIDERegs = packed record
bFeaturesReg : BYTE; // Used for specifying SMART "commands".
bSectorCountReg : BYTE; // IDE sector count register
bSectorNumberReg : BYTE; // IDE sector number register
bCylLowReg : BYTE; // IDE low order cylinder value
bCylHighReg : BYTE; // IDE high order cylinder value
bDriveHeadReg : BYTE; // IDE drive/head register
bCommandReg : BYTE; // Actual IDE command.
bReserved : BYTE; // reserved for future use. Must be zero.
end;
TSendCmdInParams = packed record
// Buffer size in bytes
cBufferSize : DWORD;
// Structure with drive register values.
irDriveRegs : TIDERegs;
// Physical drive number to send command to (0,1,2,3).
bDriveNumber : BYTE;
bReserved : Array[0..2] of Byte;
dwReserved : Array[0..3] of DWORD;
bBuffer : Array[0..0] of Byte; // Input buffer.
end;
TIdSector = packed record
wGenConfig : Word;
wNumCyls : Word;
wReserved : Word;
wNumHeads : Word;
wBytesPerTrack : Word;
wBytesPerSector : Word;
wSectorsPerTrack : Word;
wVendorUnique : Array[0..2] of Word;
sSerialNumber : Array[0..19] of CHAR;
wBufferType : Word;
wBufferSize : Word;
wECCSize : Word;
sFirmwareRev : Array[0..7] of Char;
sModelNumber : Array[0..39] of Char;
wMoreVendorUnique : Word;
wDoubleWordIO : Word;
wCapabilities : Word;
wReserved1 : Word;
wPIOTiming : Word;
wDMATiming : Word;
wBS : Word;
wNumCurrentCyls : Word;
wNumCurrentHeads : Word;
wNumCurrentSectorsPerTrack : Word;
ulCurrentSectorCapacity : DWORD;
wMultSectorStuff : Word;
ulTotalAddressableSectors : DWORD;
wSingleWordDMA : Word;
wMultiWordDMA : Word;
bReserved : Array[0..127] of BYTE;
end;
PIdSector = ^TIdSector;
TDriverStatus = packed record
// Error code from driver, or 0 if no error.
bDriverError : Byte;
// Contents of IDE Error register. Only valid when bDriverError is SMART_IDE_ERROR.
bIDEStatus : Byte;
bReserved : Array[0..1] of Byte;
dwReserved : Array[0..1] of DWORD;
end;
TSendCmdOutParams = packed record
// Size of bBuffer in bytes
cBufferSize : DWORD;
// Driver status structure.
DriverStatus : TDriverStatus;
// Buffer of arbitrary length in which to store the data read from the drive.
bBuffer : Array[0..0] of BYTE;
end;

var hDevice : THandle;
cbBytesReturned : DWORD;
ptr : PChar;
SCIP : TSendCmdInParams;
aIdOutCmd : Array [0..(SizeOf(TSendCmdOutParams)+IDENTIFY_BUFFER_SIZE-1)-1] of Byte;
IdOutCmd : TSendCmdOutParams absolute aIdOutCmd;

procedure ChangeByteOrder( var Data; Size : Integer );
var ptr : PChar;
i : Integer;
c : Char;
begin
ptr := @Data;
for i := 0 to (Size shr 1)-1 do
begin
c := ptr^;
ptr^ := (ptr+1)^;
(ptr+1)^ := c;
Inc(ptr,2);
end;
end;

begin
Result := ''; // return empty string on error
if SysUtils.Win32Platform=VER_PLATFORM_WIN32_NT then // Windows NT, Windows 2000
begin
// warning! change name for other drives: ex.: second drive '\\.\PhysicalDrive1\'
hDevice := CreateFile( '\\.\PhysicalDrive0', GENERIC_READ or GENERIC_WRITE,
FILE_SHARE_READ or FILE_SHARE_WRITE, nil, OPEN_EXISTING, 0, 0 );
end
else // Version Windows 95 OSR2, Windows 98
hDevice := CreateFile( '\\.\SMARTVSD', 0, 0, nil, CREATE_NEW, 0, 0 );
if hDevice=INVALID_HANDLE_VALUE then Exit;
try
FillChar(SCIP,SizeOf(TSendCmdInParams)-1,#0);
FillChar(aIdOutCmd,SizeOf(aIdOutCmd),#0);
cbBytesReturned := 0;
// Set up data structures for IDENTIFY command.
with SCIP do
begin
cBufferSize := IDENTIFY_BUFFER_SIZE;
// bDriveNumber := 0;
with irDriveRegs do
begin
bSectorCountReg := 1;
bSectorNumberReg := 1;
// if Win32Platform=VER_PLATFORM_WIN32_NT then bDriveHeadReg := $A0
// else bDriveHeadReg := $A0 or ((bDriveNum and 1) shl 4);
bDriveHeadReg := $A0;
bCommandReg := $EC;
end;
end;
if not DeviceIoControl( hDevice, $0007c088, @SCIP, SizeOf(TSendCmdInParams)-1,
@aIdOutCmd, SizeOf(aIdOutCmd), cbBytesReturned, nil ) then Exit;
finally
CloseHandle(hDevice);
end;
with PIdSector(@IdOutCmd.bBuffer)^ do
begin
ChangeByteOrder( sSerialNumber, SizeOf(sSerialNumber) );
(PChar(@sSerialNumber)+SizeOf(sSerialNumber))^ := #0;
Result := PChar(@sSerialNumber);
end;
end;

// for mor information about S.M.A.R.T. ioctl see
// http://www.microsoft.com/hwdev/download/respec/iocltapi.rtf

// see also sample SmartApp from MSDN Knowledge Base
// Windows Development -> Win32 Device Driver Kit ->
// SAMPLE: SmartApp.exe Accesses SMART stats in IDE drives

// see also http://www.mtgroup.ru/~alexk
// IdeInfo.zip - sample delphi application using S.M.A.R.T. Ioctl API

// Notice:

// WinNT/Win2000 - you must have read/WRITE access right to harddisk

// Win98
// SMARTVSD.VXD must be installed in \windows\system\iosubsys
// (Do not forget to reboot after copying)

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Е т'ва е живот!