No freezes here on 8.1 with avast, what actual release number do you have ? 2014.9.0.2013 is the current.
If the upgrade to 8.1 was done with avast already installed it may have gotten corrupted so you can try a repair via Programs and Features ( click to uninstall avast then choose repair on the popup screen ) if the repair fails then try a clean install of your avast.