I used following code:
$productCollection = Mage::getResourceModel('catalog/product_type_configurable_product_collection') ->setFlag('require_stock_items', true) ->setFlag('product_children', true);
But how can only select simple products that belongs to an enabled configurable product (status = 'enabled'). Above code don't care about parent product's status. I need most efficient way to do it as I have thousands of products in my store.