Thank you for taking the time to look into this for me John.
Yes, this is a business system. Basically it's just a SQL server box which runs SQL as a backend to our NPA software.
As it happens I noticed something in the event viewer about log me in yesterday so I've uninstalled that now. It used to be used by us for remote access but we no longer use that, we use simple help instead.
I cannot uninstall malwarebytes at the moment, my boss is away on holiday and I don't want the responsibility of uninstalling it in case the machine goes down with a ransomware attack again. (It's been hit twice before with these, that's why we've got malwarebytes installed.) The machine usually goes down daily but, fingers crossed, it hasn't gone down since I uninstalled log me in. As far as I can see Malwarebytes is up to date, I click "check for updates" and it says "Current". The system time is also correct. I'll free up some space on the HD and update the ethernet drivers and see how it goes from there.