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Well .asp is just active server page, meaning the content of the page is changeable dynamically by the on-line server. In your case the page has a basic layout and depending on the zip/post code entered the map data is actively imported into the page.

There should be no real difference in this to any other page as far as avast is concerned and avast doesn’t block but scans content.

I assume you mean you can’t view .aps pages rather than you can’t download them (albeit that they are downloaded into a browser cache) ?
Try this google search results page http://www.google.co.uk/search?q=cant+open+pages+that+end+in+ASP.
This is the first of many hits http://support.microsoft.com/kb/323292.