I have been following it and I'm at a loss as to what to suggest since most of what usually resolve this has been tried.
Are you installing avast from an account with administrator privileges ?
After uninstalling avast using add remove programs - When you ran the avast uninstall utility, did you run it from safe mode ?
That is the only thing that I can think of where there might be remnants of the old installation.
I'm also confused as to how you are actually installing avast as you say it doesn't even make it to the control panel but here it is trying to update
<snip>
Tech, I can't get Avast to install following your suggestion. It does not even make it to control panel, so setup.ini only contains those 2 lines:
[DownloadSettings]
FailSafeServer=C:\DOCUME~1\Serge\LOCALS~1\Temp\_av_sfx.tm~a02804
[UpdateSummary]
LastErrorMsg=Invalid file signature. Setup will terminate.
Though this was possibly earlier.
I would suggest downloading direct from avast and save it to your hard disk somewhere you can find it again, rather than try to download and install in one action.
When you have it saved to your hard disk you should be able to install from an account with admin privileges or using the run as (right click on the setupeng.exe file) administrator. But I have to admit this is clutching at straws as I haven't come across this before.