|you can begin by properly removing your previous AV like Norton or McAfee. Go to their websites and use their uninstallers...the add/remove of those will NOT clean out the registry items. If they are not totally removed from the registry no other AV will work properly.|
If those were not pre-installed on your PC then i would suggest you do a full scan with malwarebytes for starters, fix all it finds and see if that helps. It could be malware causing the problems.
You can also uninstal avast using their removal tool:
Then you can re-install a fresh version and it should work for you.
Some HELP in posting on Computing.net plus free progs and instructions Cheers