$product = Mage::getModel('catalog/product')->load($product_id);
 $site_url = Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_WEB);
 $product_url = '';
 $cats = $product->getCategoryIds();
 if (sizeof($cats)) {
     $category = Mage::getModel('catalog/category')->load((int) $cats[0]);
     $product_url = $site_url.$product->getUrlPath($category);
 }