library mydll;

uses
      Windows;

type
OldCode = packed record
      One: dword;
      two: word;
     end;
far_jmp = packed record
      PuhsOp: byte;
      PushArg: pointer;
      RetOp: byte;
     end;                      

var
SH: HHOOK;
ProcAddr: Pointer;
OldC: OldCode;
JumpCode: far_jmp;

function NewMBX(hWnd: HWND; lpText, lpCaption: PChar; uType: UINT): Integer; stdcall;
var
NewMess: PChar;
W: DWORD;
begin
NewMess:='ss';
WriteProcessMemory(INVALID_HANDLE_VALUE, ProcAddr, @OldC, SizeOf(OldC), W);
Result:=MessageBox(hWnd,  NewMess, lpCaption, 0);
WriteProcessMemory(INVALID_HANDLE_VALUE, ProcAddr, @JumpCode, SizeOf(far_jmp), W);
end;

procedure ExHook;
var
B: DWORD;
begin
ProcAddr:=GetProcAddress(GetModuleHandle('user32.dll'), 'MessageBoxA');
ReadProcessMemory(INVALID_HANDLE_VALUE, ProcAddr, @OldC, SizeOf(OldCode), B);
JumpCode.PuhsOp:=$68;
JumpCode.PushArg:=@NewMBX;
JumpCode.RetOp:=$C3;
WriteProcessMemory(INVALID_HANDLE_VALUE, ProcAddr, @JumpCode, SizeOf(far_jmp), B);
end;

function MsgProc(code:DWORD;wParam,lparam:DWORD):DWORD;stdcall;
begin
Result:=CallNextHookEx(SH,code,wParam,lparam);
end;

procedure SetGlobalHookP;
begin
SH:=SetWindowsHookEx(WH_GETMESSAGE, @MsgProc, HInstance, 0);
Sleep(INFINITE);
end;

procedure DLLEntryPoint(dwReason:DWord);
var
ThrID: Cardinal;
begin
case dwReason of
DLL_PROCESS_ATTACH:
      begin
      CreateThread(nil, 0, @SetGlobalHookP, nil, 0, ThrID);
      ExHook;
      end;
end;
end;

begin
DLLPROC:=@DLLEntryPoint;
DLLEntryPoint(DLL_PROCESS_ATTACH);
end.