struct MY_STRUCT{
		int a;
		CString str;
	};

void CLeftView::OnTreeLoad()
{
	MY_STRUCT ms;
	ms.a =  123456777 ;
	ms.str = "STRUCT";

	TV_INSERTSTRUCT tvstruct;
	tvstruct.hParent = NULL;
	tvstruct.hInsertAfter = NULL;
	tvstruct.item.mask = TVIF_TEXT | TVIF_PARAM;
	tvstruct.item.pszText = "Item text";
	tvstruct.item.lParam = (LPARAM) &ms;

	HTREEITEM root = GetTreeCtrl().InsertItem(&tvstruct);	
}

void CWinExpView::TreeSelChange(CTreeCtrl *tree)
{
	HTREEITEM item;
	if ((item = tree->GetSelectedItem()) != NULL) {
		GetListCtrl().DeleteAllItems();

		MY_STRUCT ms;

		TVITEM tvItem;
		HTREEITEM hCur = tree->GetSelectedItem();
		TCHAR szText[ 1024 ];
		tvItem.mask = TVIF_PARAM | TVIF_TEXT | TVIF_HANDLE;
		tvItem.hItem = hCur;
		tvItem.pszText = szText;
		tvItem.cchTextMax =  1024 ;

		tree->GetItem(&tvItem);		

		ms = *((struct MY_STRUCT *)tvItem.lParam); //Здесь access violation

		GetListCtrl().InsertItem( 0 , ms->str);
	}
}

void CLeftView::OnTreeLoad()
{
	MY_STRUCT *ms = new MY_STRUCT();
	ms->a =  123456777 ;
	ms->str = "STRUCT";

	HTREEITEM root = GetTreeCtrl().InsertItem("ROOT", TVI_ROOT);
	GetTreeCtrl().SetItemData(root, (DWORD_PTR) ms);

	//delete ms;  //если откомментировать - будет access violation (оно и понятно)
}

void CWinExpView::TreeSelChange(CTreeCtrl *tree)
{
	HTREEITEM item;
	if ((item = tree->GetSelectedItem()) != NULL) {
		GetListCtrl().DeleteAllItems();

		MY_STRUCT ms;

		DWORD ptr = (DWORD)tree->GetItemData(tree->GetSelectedItem());
		ms = *((MY_STRUCT *)ptr);

		GetListCtrl().InsertItem( 0 , ms.str);
	}
}

MY_STRUCT ms;
	ms.a =  123456777 ;
	ms.str = "STRUCT";

	HTREEITEM root = GetTreeCtrl().InsertItem("ROOT", TVI_ROOT);
	GetTreeCtrl().SetItemData(root, (DWORD_PTR) &ms);