Antivirus software doesn't seem to help PCs much.
It may slow down your PC initially, but it keeps the performance for life. You might have great performance after a fresh install, however it doesn't last for long.
Personally, I'd rather have a working PC than have to deal with a BSoD. I doubt I'd get many virus' and such as I don't download those kinds of files. However if you have a teenager in your family either whip them every time they try to touch a computer, but the way that's less likely to end in your incarceration is installing virus software.
The choice is prevent or protect, and I'm guessing many people here could prevent through their browsing behavior, I doubt the same could be said for any 12-16 year olds.
The trick for both is using it.
I find a more viable approach is to be careful with the OS install (not install untrusted stuff) and also to make it expendable. Data lives on a single-purpose unix host and is regularly backed up so I can do a predictable, quick, low-effort reinstall and be back to operation in a couple of hours.
An outcome I've wanted but not done - computer lives behind a router which is set up to make active monitoring and managing of traffic easy. Product idea - consumer-ready but geek-friendly router with powerful but easy-to-use tools that doesn't suck. Interface should be live so that you can see the traffic coming and going, and there should be controls to allow you to open up and close access to different sites. Stand-along system with a profile a bit like a sunray, plug for commodity keyboard and monitor.
Malware will also slow down your computer, and could result in your personal information being stolen. If the choice were between waiting an extra few seconds for my computer to boot or having all of my files accessible to people I neither know nor trust, I would choose the former.
If you're working without at least common-sense anti-virus protection, you're going to deserve what you'll eventually get.
I'd like to know what your definition of "common-sense anti-virus protection" is, though. Apart from "don't download/install anything from untrusted places".