How to tell server its 404 error for querystring value in ASP

Hi,
I have a eCommerce site in classic asp. On product detail page (product.asp), if the Product_ID value is not found in database, the asp script displays a 404 Product not found error message for customers. But the Google crawl error page mentioned following:
"The target URL doesn't exist, but your server is not returning a 404 (file not found) error"

My question is how to tell server its 404 error if specific product_id is not found in DB. The page will not automatically go to custom 404 page as the product.asp is there but the product_id value is not found.

Thanks,
Babar