I don’t know if this is your specific case, but problems with Windows Explorer are sometimes seen when other security tool was not completely removed.

Even if you ran “Add/Remove Programs” (say, for your previous AV), there might be some remnants clashing against your current Avast.

Find some removal tools at : http://thewebatom.net/uninstallers/security-software/, download the appropriate removal utility for your old security tools and reboot into Windows Safe Mode to run it.

Then run a Repair of Avast installation ("Add/Remove Programs → Avast → change → Repair). Then reboot.