Sorry I haven’t replied but Avast Forums did not email me to say there had been replies.

What exact version of Windows ? Windows 10 Version 1703 for x64-based Systems (KB4022725) 16/6/07
What exact version of avast ? 17.5.2302 (build 17.5.3559.0)
Any other security (related) software installed ? (or was there) Comodo Firewall.
Are you using a custom theme ? No.
Did you perform a repair of avast ? Yes.
Did you perform a clean installation of the latest avast version ? Yes.
Have you checked for newer drivers for the mouse/keyboard ? Yes.

Can’t suss this out. I also have an XP laptop and this doesn’t happen.