First, Update Avast via GUI to 6.0.1125.
Reboot and check.

If problem persists: try to do a repair installation of Avast via Win Control Panel.
In the list of installed programs, doubleclick Avast, scroll down on the left side until you reach “Repair”, select that and repair. Reboot and check.

If this does not solve the problem: go to Avast GUI → Settings (upper right corner of GUI) → Troubleshooting and enable the “Load Avast service only after other services…” option.
(This solves some possible boot up service starting timing problems)
Reboot and check.