This causes access issues, so unfortunately it can manifest in all kinds of ways, especially given that AVG can choose different folders for different users / computers.

In the specific I case I saw, ProSeries was failing to update the 1040 and 1120 products repeatedly (because AVG had protected the 1040 and 1120 folders). 

The only time updates were successful was when we restarted ProSeries or ran the updater with Windows in safe mode, which indicated there was some other program interfering with ProSeries.