Make sure the date and time is correct on your machine first.
You can try an Avast Repair:
XP Users:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
-
Scroll down and choose Repair function in the pop-up window.
- Reboot.
Vista & Win7 Users:
- Control Panel
- Programs & Features
- Uninstall a Program > Select Avast Antivirus
- Click the Change/Remove button and
scroll down to Repair
- Click Next and follow.
- Reboot.
If this doesn't work, then since you did an upgrade, an uninstall/clean install might be in order:
1. Save a copy of Avast and save it to your HDD:
Avast Internet Security:
http://files.avast.com/iavs5x/avast_internet_security_setup.exe2. Download the Avast Uninstall Utility
http://www.avast.com/uninstall-utility and save it to your HDD or desktop.
3. Disconnect from the Internet at this time.
4. Go into
Safe Mode (hit F8 repeatedly) and run the Avast Uninstall Tool.
Uninstall ALL previous versions and products of Avast you had on this machine and reboot in between each uninstall. 6.
Reboot.
7. Install the newest version of Avast and
reboot.
8. Get Internet access and add the license key for AIS online.
9. Update the Avast definitions.