same here, i experienced this issue with two titles:
-warhemmer 40.000 space marine demo
-deus ex: human revolution
i am not comfortable to turn off avast! to be able to install these especially that this final game setup needs active internet connection.
interestingly enough i couldnt find anything that avast! had blocked, empty virus chest, steam folder in exclusion list etc...
im not convinced that this is an avast! problem, these two games setup differently from those in the past: earlier steam downloaded everything you needed into the game folder (directx, vcredist etc) and then before first launching the game it installed these from this offline source.
now with these titles steam also uses your internet connection so i guess it (partly) installs these from online source or at least checks if these redistributables are up to date.