I was torn between this program and AVG anti-virus. I'm guessing both Avast and AVG are almost equally good, but heard slightly better things about this product, so I picked it. I've liked using this product. It seems to update the virus definitions daily. It doesn't slow down my machine, and I haven't noticed flaking behavior with other products. If you use it for personal use the product is free, otherwise it costs around $40 with volume discounts available and discounts for multi-year purchases.
The only thing that annoys me is that the program does give false positives for many email messages with peculiar whitespace. Worse yet, it talks and there's no way to shut it up that I could find. I wish I could tell it not to issue a spoken warning.
So if you're looking for a good anti-virus solution, Avast would be a fine choice.