Norton and McAffee are notorious for not stopping known threats...and being resource hogs. If I had a dollar for every system I've had to do a complete reload or virus removal on, that was running either system, I'd own a Hummer right now. Both corporate and home user versions are proven to be easily shut down by many known viruses and malware. Tren Micro's PC Cillin is a little better, but still a resource hog.
If you're not comfortable with freeware versions of anti-virus get a paid version of Avast, Avira Antivir or Nod32...then supplement with Spybot Search and Destroy (immunization, tea timer, IE hosts features)...as well as WOT (Web of Trust)...along with not opening suspect emails/attachments...you're system will stay pretty safe.
Otherwise, stay off the russian and chinese domains, no P2P file sharing, and don't surf the porn sites...or else be ready to completely reload your computer.