Hi
Saw this post while googling why my WOW latency was so high, and it seems that AVAST was causing it. The most simple thing to do is to just make sure nothing is accessing the internet while you play WOW and to simply disable avast firewalls while you're playing. You can do this by right clicking the icon in the tray, selecting avast! shields control and then choosing how long you want to deactivate the firewalls. I noticed a huge drop, where I was previously sitting on a latency of 300ms - 3000ms I'm not at 40ms-90ms.
Hope this helps!