Perhaps this is not a solution either, and is somewhat potentially risky, but has anyone tried adding the Left 4 Dead 2 game folders to avast!'s File System Shield Exclusions list? (See attached image.)
The computer will still be protected against threats, it just won't be directly scanning any Left 4 Dead 2 files or folders. If a threat did happen to come along, it would still most likely be picked up by another avast! Real-Time Shield. Again, as previously stated, I must stress that this is a little bit risky.
There has been mention of more Real-Time Shields than just the avast! File System Shield causing problems, so maybe this won't work. However, I reckon it's worth a try.