DavidR
2
You could start with some information.
What Operating System are you using ?
How many users and are they limited account users or do they have administrator rights ?
Check the avast! Log Viewer (right click the avast ‘a’ icon), Warning or Error sections, this should contain information on errors/warnings, if you can post the full info about this error message.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
If so don’t do anything else until you give is the information about it.
I would suggest uninstall and run the uninstall utility also then reinstall from ‘the administrator’ account not just one with admin rights.
Download the avast! Uninstall Utility, find it here and save it to your HDD. Now uninstall, reboot, run the uninstall utility, reboot install avast from the administrator account, reboot.