Delphi API Hook的问题

Delphi(Pascal) code:

{一个修改IAT实现API挂钩的程序,}
library Hook;

uses
Windows,
w_Hook in 'w_Hook.pas';

{$R *.res}

function DllEntry(dwReason:LONGINT):DWORD;STDCALL;
begin
case dwReason of
DLL_PROCESS_ATTACH:
begin
w_Hook.IntallHook;
end;
end;

result := 0;
end;

begin
DllProc := @DllEntry;
DllEntry(DLL_PROCESS_ATTACH);
end.

/////////////////////////////////////////////////////////////////////////


unit w_Hook;

interface
uses
Windows,SysUtils,Classes;
type
PIMAGE_IMPORT_DESCRIPTOR = ^IMAGE_IMPORT_DESCRIPTOR;
IMAGE_IMPORT_DESCRIPTOR = record
OriginalFirstThunk : DWORD;
TimeDateStamp : DWORD;
ForwarderChain : DWORD;
Name : DWORD;
FirstThunk : DWORD;
end;

PIMAGE_THUNK_DATA = ^IMAGE_THUNK_DATA;
IMAGE_THUNK_DATA = record
Function_ : PDWORD;
end;

procedure IntallHook;STDCALL;

implementation

function MyMessageBox(hwnd:CARDINAL;lpmsg:pchar;lpcaption:pchar;nType:UINT):dword;stdcall;
begin
result := MessageBox(hwnd,'HOOK','HOOK',MB_OK);
end;

procedure IntallHook;STDCALL;
var
OpHeader : IMAGE_OPTIONAL_HEADER;
NtHeader : IMAGE_NT_HEADERS;
dosHeader : IMAGE_DOS_HEADER;
ImpHeader : PIMAGE_IMPORT_DESCRIPTOR;
hModule : CARDINAL;
thunk : PIMAGE_THUNK_DAT