Is the time and date correct on your machine?
Have you tried an Avast Repair:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
- Scroll down and choose Repair function in the pop-up window.
- Reboot.
Let us know if this fixes your problem. If not, please let us know what other security software you have on this machine (antivirus version, firewall, other security software - currently and in the past and how you uninstalled them). Thank you.