It causes the infamous crashes with sound looping. It's actually an issue with the way Windows handles paged kernel memory, and having antivirus software resident seems to mean it runs out. The suggested fix I've seen (other than uninstalling AVG) is to increase the size of the PagedPool:
More info on this thread - I've increased my pagedpool to the maximum size and it made my crashes go away, even with AVG still running.
Unfortunately, because of the way antivirus software has to be kept resident, it's unlikely that there's going to be an easy fix for the antivirus software authors, short of trying to reduce their memory footprint.