Some files cannot be scanned. the files are usually archive files and are by nature inert. The files are not executable files. I am taking it for granted this is a custom scan. If so, click packers and leave unchecked the all packers box. For more info check out the default scan settings. I had the same problem until I unchecked the all packers box. Since then I have had no problems. Hoped this helped.
Dell Inspiron, Win7x64 SP1, Pentium Dual-Core, 6 GB Ram, Avast Free 2014.9.0.2018, Comodo Firewall 5.12 w/D+, MalwareBytes Premium 2.0, MCShield, Super Anti-Spyware Free, SpywareBlaster, Bitdefender TrafficLight, OpenDNS Premium, Keyscrambler Personal, PrivDog. CCleaner, Greenshot, Firefox (latest build) and, Google Chrome (latest build).
When you do something, do it with a purpose and do it on purpose.