すんません。CM_MOUSEENTERです・・・

private:// ユーザー宣言
void __fastcall OnMouseEnter(TMessage & msg);
void __fastcall OnMouseLeave(TMessage & msg);
public:// ユーザー宣言
__fastcall TForm1(TComponent* Owner);

protected:

BEGIN_MESSAGE_MAP
VCL_MESSAGE_HANDLER(CM_MOUSEENTER, TMessage, OnMouseEnter)
VCL_MESSAGE_HANDLER(CM_MOUSELEAVE, TMessage, OnMouseLeave)
END_MESSAGE_MAP(TForm)