I experience this as well. Wish it would do like you said instead of stopping.
Actually, there's a reason for this. I have occasionally found this annoying when, for example, I was copying files to it over the network from an untested customer's computer as a backup, and Avast found a booger, and consequently stopped the whole computer from doing anything at all. The network file transfer that I had intended to have finish overnight hung up. I was a bit upset.
I later learned the reason for it. A client of mine was using AVG (yuck! gag! barf!) and got a notice from AVG that a virus had been detected. Before she could finish reading the message, the virus realized it was busted, and shut down the computer.
Avast freezes the computer to be "in charge" more than the virus. It's true that this behavior has its downsides, but equally true that it is worth the irritation.
By the way, when Avast 5.0 is "finally released" it will be kick-butt like all of the Avast products, because Alwil doesn't rush stuff onto the market prematurely like Micro$oft and most of their buddies. Avast sings with Mr. Roger, may he rest in peace: "I like to take my time to do it right." Long live the memory of Fred Rogers. Even if you couldn't handle his extreme calm, he was undeniably one of the kindest and "best" to walk this earth.
So be patient. It'll be released when it's completely wonderful. And you'll love it.