Here we say "Buy a Dell, end up in hell" 
Much better than a factory reset is a clean installation with a Windows cd/dvd.
Guess you don't know that IObit can't be trusted.
They stole from Malwarebytes.
I've probably had 3 Dells die on me now... though this one has been OK until now. Which make do you prefer?
It came with Windows pre-installed so I don't have the CD/DVD. I kept cancelling the upgrade to Win 10 but am wishing I did it now...
Didn't know that about IObit - can't remember where I got it from - CNet?
I'm hoping essexboy (or anyone!) can help me sort this out without a reinstall... The computer was fine until the Avast update (2016?). I clicked restart as Avast needed to restart my computer and then it was caput... Perhaps it corrupted the registry? I've seen many people post on the internet with exactly the same problem after updating Avast. Can the registry be fixed?