I used to use Avast but changed over to AVG, so I'm sorry I can't help with the pop-up warning. - AVG has no effect on that page, but I can confirm that the Buy Now link is disabled. - It shows an, 'Error on page' in the Status bar, when 'Buy Now' is clicked.
It's not really disabled per se. For whatever reason, there are two links on that page to "Buy Now" which try to use an odd method of redirecting to:
http://www.serenescreen.com/product/...t.php?mode=add
Instead of a regular link, they are trying to use javascript to activate a form submission to do the redirection. Very odd indeed.