AVG flagged the "Player Behavior" app yesterday. But... I've been a good girl, Rito. 
Seriously, effers who are going to come here and troll you for using a "crappy" antivirus need to lick my toenail. Avast is a perfectly competent free program. I have a relative who works for AVG and I've been using the full product for free for many years across three systems. This is also a product that hiveminded good ol' boys flag as "crap." It uses very little resources and has protected me from a total of maybe 5 threats because I browse the internet intelligently (except for 5 times.)
If you're not an idiot, all you need is Windows' built-in firewall and antivirus and maybe an occasional MWB scan to make sure you're doing okay. Use adblock, popup blocker, and don't click on stupid links and you can browse all the free porn you want with peace of mind.
Anyway, just know that if anything truly (and I'll say exclusively, to negate any PMB arguments) malicious was lurking in the world's biggest online game, it would be breaking news. I'd assume with the new patch, files have changed and have not been analyzed yet by the Avast team. Legitimate programs can be flagged because they exhibit behavior that is either typical of malicious programs or is unusual for the file type. It's not much to be ashamed of-- it's kind of like an allergy. Yeah, it's annoying and wrong, but at least you your "immune system" is working. :)
I would just double-triple check that you're not being phished, and that you were indeed browsing http://boards.na.leagueoflegends.com/en/. Otherwise, it could've been a "true positive." Ha.
Lastly, try contacting Avast at the following link to report the false positives so they can improve:
https://www.avast.com/contact-form.php?subject=VIRUS-FILE
