I use the free Microsoft virus protection . better protection, for bupkis .
plus Norton slows down your computer
I started using MS Security Essentials on my previous XP desktop and laptop. Other security programs got to be bloated, slow, and conflicted with other programs.
When I finally got a new laptop and desktop with Win7, I went MS Security Essentials on both.
I use a program called Sandboxie to run programs (browsers, mainly) in a security box. Several virus programs had difficulty with Sandboxie, but MS Security Essentials works fine with it.