Q: Error message from Antivirus

I DON'T run antivirus on my Mac.

See: https://discussions.apple.com/message/23885605#23885605

If you feel the need to use Avast! or other AVware on your Mac, then you should contact the makers support site and have them sort out their problem(s).

avast! Free Antivirus for Mac | Security Software for Apple OS X

Personally I have never used any AVware on any of my Mac's. For more on the subject, please check out the following links including others in the "More Like This" box just to you right.

Mac maintenance Quick Assist

The Safe Mac » Mac Malware Guide

"Avast" is perhaps the worst of the whole wretched lot of commercial "security" products for the Mac. It's worse than the imaginary "viruses" you were worried about when you installed it. Not only does it fail to protect you, it throws false warnings, destabilizes and slows down your computer, and sometimes or always corrupts the network settings and the permissions of files in your home folder. Removing it may not repair all the damage, and neither will Disk Utility or even reinstalling OS X.

Back up all data, then remove "Avast" according to the developer's instructions. Reboot.

If you tried to remove Avast by dragging an application to the Trash, you'll have to reinstall it and follow the instructions linked above.

If you still have problems after following those instructions, post again.