Had same problem today on a Windows 8 machine... McAfee had been uninstalled, so I ran their uninstall tool as well, and ran the Norton uninstall/cleanup tool just in case there might have been some remnants of that from long ago. No luck. Finally I installed and ran Malwarebytes free version via full scan, and after removing various PUPs (no virus files detected) and reboot, Avast installer ran perfectly! Hope this helps someone with similar problems...