Before I retired from the USN in '06, both Norton and McAfee were available for free download, as long as you had a valid DoD PKI Certificate on your computer. I was even able to download them at home after installing my certificate. Alas, the certificate expired after I left the service, so I can't access that perk anymore.
But, as an employee of the University of Illinois, I can get McAfee for free here. Prior to working here, I was using AVG Free and it worked great. I'm now using McAfee, and it's working well so far. I've used both applications on XP and Vista now with no problems.