McAfee si, Norton no.
I don't normally discuss techie stuff; arcane debates about one product versus another tend to be tiresome and useless, though they do fill up a lot of bandwidth on all web-based forums.
However! This was such a dramatic difference that it's worth 'registering' for the sake of searches.
I've been using Comcast as my ISP. For the last two years Comcast included McAfee antivirus as part of the package. I was completely happy with McAfee; it stopped all viruses and malware, and it stayed out of the damned way better than other antivirus systems I'd used before.
Last week Comcast switched its free service from McAfee to Norton, and I went along because it was still free.
Immediately Norton got underfoot like a cat, slowing everything down, thrashing the disk for long periods with no obvious reason, and popping stupid warnings about "failing to back up."
This afternoon Norton
wouldn't let me shut down the computer. Two of its component DLLs were busy and couldn't be halted.
When an "antivirus" causes as much trouble as a virus, it's worthless.
Went to McAfee and bought their yearly package, tossed Norton and reinstalled McAfee.
All is well again.