I registered on these forums specifically to post in this thread checking to see if there was anything in these forums about this issue (the incomplete installation (55) issue). I'm experiencing it with Deus Ex: Human Revolution, which just unlocked this morning, which is to say that it tried to unlock, but Avast somehow blocked it without any of its scanners showing anything suspicious. It sat on 1% for several seconds before declaring an incomplete installation (55).
I ended up disabling avast, which allowed it to unlock, and then it unlocked (I didn't try disabling avast while I was playing, so I don't know if it would have run faster without it). About a half hour ago, I closed the game, and it tried to install a patch (Avast was active). It downloaded it, but failed to install, with the same problem as before. Since the patch didn't work I've tried to make it work without disabling avast, but without much luck. I've had to disable the file shield, P2P shield, and IM shield, and trying to let steam patch it without disabling the file shield was impossible, even with an exclusion on the steam folder (H:\Program Files\Steam\*) in the file shield's expert settings -> exclusions (with r/w/x all checked). However I noted using AnVir task manager that Steam appeared to be running a DLL file which was not named with a .dll extension (~26.tmp) from my Documents and Settings\[username]\Local Settings\Temp folder. It exports one function, _runfunc@20, and imports a lot of functions, which doesn't give me any idea what it's about. Putting an exclusion on that file specifically wouldn't be possible if it's recreated whenever steam is run (why are they doing that, anyways?), and an exclusion on the temp folder would be downright dangerous.
Another thing. Approximately 80% of the games I've gotten from Steam in the past several months (including from the Humble Indie Bundle 3, quite a few) were thrown by Avast into the autosandbox automatically, until I went looking for the setting to make it stop doing that automatically, and I've seen some annoying false positives which were never addressed before too (such as seeing a program which came with HP computers and was a vital part of the automatic system restore software, be classified by Avast as a 'potentially unwanted program', and as such automatically getting thrown into the virus chest).
P.S. I'm using Windows XP Home SP3 and Avast 6.0.1203.
