How should I protect my Windows PC from malware and viruses?

Gwilym isn't convinced his anti-virus software is worth the money, and wants to know what protection I'd recommend

I am using Avast to protect my computer: you recommended it quite some time ago. I am not entirely convinced it's worth what I am paying, and it is constantly suggesting, in a variety of less than subtle ways, that I should upgrade. What protection would you now recommend? I am prepared to pay for something that works. Gwilym

It's complicated. I've spent more than 20 years recommending various anti-virus programs as an essential part of any Windows setup. However, Windows has changed, and the threat landscape has changed. I am no longer sure that a third-party AV program is essential, and some of them may be detrimental.