Hello Moyners,
Avast detects conflicting software based on KEXTs (drivers) loaded into kernel. You can list the drivers by opening Terminal.app and typing 'kextstat'
If there is any of the following IDs in the listing, it is detected as Kaspersky:
"com.kaspersky.kext.klif"
"com.kaspersky.nke"
"com.kaspersky.kext.kimul.44"
"com.kaspersky.kext.mark.1.0.3"
If you have correctly uninstalled Kaspersky but the driver remained there, a reboot should resolve the issue. Alternatively you can run in Terminal.app:
sudo kextunload -b <id of the bad kext>
This will try to unload the driver without rebooting. You can check with kextstat whether it worked.
If you can't install Avast even after removing all of Kaspersky drivers, please provide us with the output of kextstat command to take a closer look.
Best regards,
Jakub