It happened to me, too, and I wanna play League of Legends, so I simply added Riot Games' folder to exceptions list of file scanning Module.
Take a look at LoL's forum, and at this thread I posted how I repaired the error that happens after the detection: <
http://na.leagueoflegends.com/board/showthread.php?p=31419020&posted=1#post31419020>.
Maybe it's being used a old version of "Xiff.dat" that could be used as some kind of exploit by malicious software, or maybe it's a real False Positive. I believe in both cases, we can't be sure, but the thing is that I wanna play the game and I won't let avast! block it by now... ;cD
And maybe later the game will update its files to not be detected. Or not, making it being detected forever. The last case would not be very confortable. Anyway, it's an official and very used software, by Riot Games, and it's almost sure it's not a virus, but a False Positive.