Hello paulb106,
Try repair of avast:
1. Go to Control Panel and open "Add/Remove programs".
2. Find "avast! Free Antivirus" and click "Change/Remove" button.
3. Choose "Repair" and click next button.
4. After finishing it reboot your PC.
If this way does not work, please add more details about your problems and computers (OSes, avast's version etc.)
Thanks.