// Header에 포함시키시고용..
private:    // User declarations
    void __fastcall AppMessage(tagMSG &Msg, bool &Handled);


// Cpp에다가 포함시키시면.... 되겠죠

void __fastcall TForm1::FormCreate(TObject *Sender)
{
    Application->OnMessage = AppMessage;
}
//---------------------------------------------------------------------------

void __fastcall TForm1::AppMessage(tagMSG &Msg, bool &Handled)
{
    if(Msg.message == WM_MOUSEMOVE){
        POINT p;
        GetCursorPos(&p);
        if(PtInRect(&(Memo1->ClientRect),Memo1->ScreenToClient(p))){
            if ( !Memo2->Visible )
                Memo2->Visible = true;
        }else{
            if ( Memo2->Visible )
                Memo2->Visible = false;
        }
        Handled = true;
    }
}

어디서 퍼온것입니다.

^_^;;; 좋은 하루 되시길..