My Windows 8.1 clients, after Avast install, come up with a massive number of startup errors. Example:
SMSS.EXE
The application was unable to start correctly (error 0xc0000142). Clock OK to close.
It then repeats dozens of times with various programs, such as WINLOGON.EXE, CMSS.EXE, etc. After you get through all the OKs, the system runs normally. This behavior does not happen on my Windows 10 clients.
Use the support link or live chat option in the portal to create a ticket on this one, we probably need to collect logs from a device or 2 to see if we can find the issue.