From Avira's website:
Currently, the Avira products are not ready for Windows 8 and Windows Server 2012 (Built on Windows 8).
Avira is working closely with Microsoft to achieve compatibility for the products as soon as possible. Therefore, it can be said with certainty that the Avira products will be compatible with Windows 8 in the first quarter of 2013.
This same information should be shown to you when you click the "Check for solutions online" button after it fails to install on Windows 8 [source].