I am a avast 5 free user i dont have an internet connection own but i like using avast from 4.8 version, i update my antivirus by using vps update (32 MB) file but after first update manual avast.setup file dissapear from avast installation folder i could knew it because i use another free tool, it showed me the executable modification.
So…? 
Shouldn’t be a problem.
It is normal to disappear after the update
Well, on avast 4.8 antivirus version that file stay (avast.setup) at avast folder installation after any manual update i always saw it but in this new version i receive an error message and i can that is this the reason. i took an screenshot about this fail on installation process.
Try a repair of avast. Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.