Antivirus software doesn't always stay good, what was good Antivirus software a couple of years ago can sometimes cause problems with systems and go out of vogue, or perform badly. My current favorite is Avast as it has a free version that is great, but if you want to do banking and purchasing online, you should pay for the full blown version, which with Avast is reasonably inexpensive when compared to the others. I dislike Antivirus packages that try to be too Clever, and weld themselves into your operating systems as Norton and Kaspersky antivirus packages have a habit of doing, since they become a third party part of your operating system, when a virus attacks them, or an update goes bad, your entire O.S. Can be destroyed. I much prefer packages that just sit on the outside of your O.S. And check for viruses and malware.
I've seen many systems where an update to Norton has destroyed their entire O.S. And a complete wipe and reload of their O.S. Was necessary. Antivirus packages should not try to be too Clever, just do what they are purchased for.