I'm still having this problem. After Avast's program updated the other day, trying to launch Left4Dead 2 would present me with a screen saying "Completing Installation... 1%" - despite the fact I've had the game installed for over a year and a half.
After hanging on that screen for several minutes, it would eventually error out with "incomplete installation". Looking up the problem on Steam's knowledge base, they mentioned McAfee caused conflicts, so I selected to turn Avast's shields off for ten minutes and the "Incomplete Installation" problem went away.
Ten minutes later, when Avast turned itself back on, I was in the middle of a Left4Dead match with my friends and suffered horrible bouts of stuttering - the game would often lock up for minutes at a time. Alt-tab, turn Avast off, stuttering vanishes.
The problem is, doing this requires leaving my entire system insecure. On top of that, me and my friends play LOTS of Left4Dead, and it's a fairly big group of friends at that, so when the call goes out that we're playing, it's often a game of musical chairs as to who gets in. Taking that extra moment to disable Avast every time I get an invite has already cost me my spot more than once since this started happening.
So either I turn Avast off forever (and then what's the point in having it installed at all?) or... well, I guess there is no other option, is there?