We have one license but use Vipre for all of devices. We purchased an unlimited home use license. I think a single use license was like thirty bucks or so, and the unlimited was like fifty if memory serves me correctly. That's quite a bargain. We use it for my desk top and my wifes desk top. Both of our laptops, and my wifes notebook, and my son's laptop. That's six devices.
It uses very little space, so your computers speed is not compromised as a result. Most of the time, I barely know it's even there. There are others that claim to do other things, and I dont think there is one that is the best, but I have been happy with Vipre.
I first learned about Vipre from one of the IT guys at Random House. He used to work for the FBI's Cyber Crimes unit, and is RUMORED to have been quite the hacker back in the day. He's a close to anyone I've ever met that was a genuine hacker. (I don't run in that circle, and am NOT very tech savvy). Anyway, I kept having problems a couple of years ago, and he hooked me up with two things: 1) Vipre 2) Some other software I had on my last computer, the name of which escapes me at the moment, but the desktop icon for it was a bundle of TNT. It was like "______this" or something. It was a very heavy hitting program that I only let my IT guy ever use because if you werent careful you'd wind up removing shit you needed. I'll think of it and let you know.