Just updated from 4 to 5. Everything SEEMS to be running OK, but I can’t access the User Interface. When I try, I get the following message:-
The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log for more detail
What’s going on, and where’s the event log?
And also there’s no icon in the tray.
Hi Paul D: Could you please provide the OS you are using and any other security software running on your computer. Also have you tried rebooting again? 
If the previous suggestions don’t solve the issue then I would suggest you do a ‘Clean Install’ of Avast5.
As a starting point, lets ensure that all remnants of Avast 4.8 are gone.
For a clean Uninstall/Reinstall please follow these instructions (I have assumed you are using the free version):
1>Download the latest version of Avast, 5.0.677 Free AV: http://files.avast.com/iavs5x/setup_av_free_eng.exe (English only) and save it to your HDD, somewhere you can find it again (if you didn’t save your last download). Use that when you reinstall.
2> Download the Avast! Uninstall Utility, aswClear5 at: http://public.avast.com/~forejt/aswClear.exe and save it to your HDD (it has uninstall tools for both 4.8 and 5.0).
• 1. Now uninstall Avast! (using add remove programs, if you can’t do that start from the next step), reboot. When your system is up and running Reboot into ‘Safe Mode’
• 2. run the Avast! Uninstall Utility from safe mode separately, first for 4.8 if previously installed, then reboot into ‘Safe Mode’, and then for 5.0. Once you have completed, reboot into ‘Normal Mode’.
• 3. Install the latest version and reboot.
This usually has something to do with C++
See here>>http://forum.avast.com/index.php?topic=60872.msg514523#msg514523
Thanks everyone - all better. All it took was steps 1 & 2 from Peetah’s post #2 in this thread:-
Paul_D, since your problem is now resolved, please go back to the first open post in this topic, click the modify button in that Post and change the title/subject, add [Resolved] to the beginning of the title so this thread can be closed.
Feel free to come back any time you need help, to learn something new, or just to ask questions. We are here 24/7 for your convenience. Thank you.