John just upgraded to Windows 10, does he still need an antivirus? His tech people say he should. Leo says it's not bad to have an antivirus, but Windows 10 already comes with one called Defender. So he won't really need anything else. But Leo also says that the best defense is good behavior. No antivirus is 100% effective...at best they are 50% accurate. Also, they don't guard against so-called Zero-Day attacks. So avoid clicking on links or opening attachments, and keep the OS updated.