>Do you find that on Windows 10 you need a commercial AV (vs built-in Defender)? On Windows 7 I use AVAST. Support sucks but it appeared not to let any viruses through. Do I need to buy it for Windows 10 too?

I used to use Avast as well and have now been using Win10 Defender for at least a year. No problems. Sometime back MS added a new feature to Defender to block programs from accessing the hard drive. I had to enable this feature and then give the permissions to the programs to access. This feature is very good. It might have been around before I knew about it but I started using it sometime in the past year. It is referred to as 'Ransomeware Protection' - you have to enable 'Controlled Folder access'. You can give the specific folders to protect.