在.h中：
public: // User declarations
    BEGIN_MESSAGE_MAP
       MESSAGE_HANDLER(WM_HOTKEY,TMessage,OnHotKey)
    END_MESSAGE_MAP(TForm)
    void __fastcall TMainForm::OnHotKey(TMessage &Message); //响应热键消息

在.cpp中：
void __fastcall TMainForm::FormCreate(TObject *Sender)
{
    //注册系统热键
    if(!RegisterHotKey(Handle,2000,MOD_CONTROL,VK_F12))
        //这里注册的热键是Ctrl+F12
        ShowMessage("热键注册失败");
}
//---------------------------------------------------------------------------
void __fastcall TMainForm::OnHotKey(TMessage &Message)
{
    //添加你的代码
    TForm::Dispatch(&Message);
    if (!MainForm->Visible)
    {
        MainForm->Show();
        MainForm->BringToFront();
    }
    else
        MainForm->Hide();
}
//---------------------------------------------------------------------------
void __fastcall TMainForm::FormClose(TObject *Sender, TCloseAction &Action)
{
    UnregisterHotKey(Handle, 2000);//退出程序时注销热键
}