First let me say thank you for your continued interest.
I don't think the buffalo unit has anything to do with the problem. The nas service is a TSR waiting for connection I just don't usually have the box itself switched on. I have tried with the linkstation box on and off and have stopped the service in task manager without making any difference to Avast. Further the Avast problem is much more recent than the linkstation - they have happily coexisted up to now.
I did ask why avastui.exe continues to be shown running in task manager, after closing the control centre by clicking on the x in the top right hand corner, and seeing the window close. avastui is not shown running until I click on either the tray icon or the desktop icon.
No I can't see or find any form of malware either - I'm sure it's something between my implementation of Dell & Windows, which I have tried to make as plain vanilla is possible, and the current Avast, which didn't happen over the last couple of years. I haven't made any changes to the system recently.
Since the installation is so new and gets cleaned up and defrags pretty regularly I can't see this as a solution. In any case an AV worth its salt should be able to run in much more congested systems.
regards