solution i used in windows 7 (edit : finally won't work)
- restart in safe mode
- use the avast uninstall tool
- restart in safe mode
- in command prompt search for all avast files (dir avast*.* /s), if find delete them in files explorer
- restart in normal mode
- install avast (free version) with offline installer
- restart in normal mode
- update virus base if needed
Launch battle.net and overwatch, everything work fine
Edit : NO THAT WON'T WORK
on first launch of battle.net + overwatch all work fine
but after leaving battle.net + overwatch, and launch them again
the problem is coming back, no access to servers
(sorry english not my native language)