I fixed this issue last week. It is the "upgrade" over exisiting avast!, vs. removal of existing avast, prior to installation of EPS. I wrote this article to deal with this issue several years ago. I was hoping that this, or similar process, would be built into the installer (automated removal of previous clients)
This article has been modified for your specific issue
1) Start Windows in Safe Mode (F8 key before Windows splash screen on a reboot)
2) Open (execute) the uninstall utility aswclear.exe
3) Choose which version of avast! to uninstall. This process will be repeated for each avast! version (4, 5, and 6) So let’s assume we had all 3 versions of avast! installed at some point. So we choose version 4.8 and proceed.
4) If you installed avast! in a different folder than the default, browse for it. (Note: Be careful! The content of any folder you choose will be deleted!)
5) Click REMOVE, close aswclear, and open aswclear, and remove avast! 5, DO NOT reboot, and repeat this process for all versions of avast
6) In Local disk C:, ProgramFiles, delete “avast” and “Alwil” directories if there. Look here for the other A/V vendors, that you will also have to deal with on a similar basis
7) In C:\Windows\temp, remove all.
Start, run, type “%temp%” and remove all.
9) Reboot
10) Install avast! and reboot!
J.R. Guthrie