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 T4500, 6 GB Ram, Avast Free v2015.10.2.2211, Comodo Firewall 5.12 w/D+, MalwareBytes Premium 2.0, MCShield, Super Anti-Spyware Free, SpywareBlaster, OpenDNS Premium, Keyscrambler Personal, Ghostery. CCleaner, Greenshot, Firefox (latest build), Google Chrome (latest build) and, various browser security tools.
When you do something, do it with a purpose and do it on purpose.