Uninstall Avast. Third-party AV software are prone to warn you of false positives. There are no viruses affecting OS X at this time. Anti-malware software is pretty much not needed.
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.