核心编程上一个关于API拦截的例子，出了点问题，求解

#include "stdafx.h"

#include <Windows.h>

#include <iostream>

#include <cstdlib>

#include <Dbghelp.h>

#pragma comment(lib,"Dbghelp.lib")



using namespace std;



VOID ReplaceATEntryInOneMod(LPSTR pszCalleeModName,PROC pfnCurrent,PROC pfnNew,HMODULE hModCaller)

{

	ULONG ulSize;

	PIMAGE_IMPORT_DESCRIPTOR pImportDesc = NULL;

	__try

	{

		pImportDesc = (PIMAGE_IMPORT_DESCRIPTOR)ImageDirectoryEntryToDataEx(hModCaller,TRUE,IMAGE_DIRECTORY_ENTRY_IMPORT,&ulSize);

	}

	__finally

	{

	}

	if(pImportDesc == NULL)

	{

		return ;

	}

	for(; pImportDesc->Name ; pImportDesc++ )

	{

		PSTR pszModName = (PSTR)((PBYTE)hModCaller + pImportDesc->Name);

		if(lstrcmpiA(pszModName,pszCalleeModName) == 0)

		{

			PIMAGE_THUNK_DATA pThunk = (PIMAGE_THUNK_DATA)((PBYTE)hModCaller + pImportDesc->FirstThunk);

			for(; pThunk->u1.Function ; pThunk ++ )

			{

				PROC* ppfn = (PROC*)&pThunk->u1.Function;

				BOOL bFound = (*ppfn == pfnCurrent);

				if(bFound)

				{

					if(!WriteProcessMemory(GetCurrentProcess(),ppfn,&pfnNew,sizeof(pfnNew),NULL) && (ERROR_NOACCESS == GetLastError()))

					{

						DWORD dwOldProter;

						if(!VirtualProtect(ppfn,sizeof(pfnNew),PAGE_WRITECOPY,&dwOldProter))

						{

							WriteProcessMemory(GetCurrentProcess(),ppfn,&pfnNew,sizeof(pfnNew),NULL);

							VirtualProtect(ppfn,sizeof(pfnNew),dwOldProter,&dwOldProter);

						}

					}

					return ;

				}

			}

		}

	}

}



int MyMessageBox(HWND hWnd,LPCSTR lpText,LPCSTR lpCaption,UINT uType)

{

	MessageBeep(0);

	return 0;

}



int _tmain(int argc, _TCHAR* argv[])

{

	PROC pfnOrig = GetProcAddress(GetModuleHandle(_T("User32")),"MessageBoxA");

	cout << GetLastError() << endl;

	HMODULE hModCaller = GetModuleHandle(_T("Test.exe"));

	ReplaceATEntryInOneMod("user32.dll",pfnOrig,(PROC)MyMessageBox,hModCaller);

	system("pause");

	return 0;

}