At one point, I got a virus warning on Ebay with Avast. Ebay, of all places! After about a day, it went away as the page changed. My stance is that I would rather have a few overzealous alerts, than have any AV miss something. And heck, maybe Ebay was infected with something.....
Also, Nimda infected a LOT of HTTP servers a while back, it is entirely possible there is still some trace of the virus left on Rotowire's server that Avast is picking up.
But to help with your question, can you put a URL in an exceptions list to make Avast not scan this site? I know you can do so with folders on a hard drive, but I'm not sure how this would work with URLs on the Web Shield.