Every A-V misses some viruses, as you'll note if you read the reviews and stats. AVG has slid down in the ratings, but Avira and Avast are still near the top with (the paid) Norton. That's why there are free online scans to doublecheck our security.
Try checking out virtualdr. com. and other forums, and you'll see that problems with installing v.5 are rife, to say nothing of the other v5 builds that preceded it. Avast shouldn't have released a version that has so many problems with so many users. Upgrades should run smoothly, and play nicely with other security apps, unless there's something seriously amiss in the user's computer.