I've got 2 PC's.

One runs XP, and has MacAfee Internet Security installed. In general this is unobtrusive but I've de-installed the Spam Filtering; it's a real resource hogger when you fire up Outlook and not stunningly accurate; I can do just as well using Outlook Rules. Prior to this I used Norton AntiVirus, which worked fine but definately hit performance.

My other machine runs Win98 that I use primarily as a fix / diagnostic platform; thats been running the free versions of AVG and ZoneAlarm for over a year. I can share the hard disc and so virus scan it using the MAcAfee system over my network, this may be slow but so far MacAfee has not identified anything on the Win98 machine that had not already been identified and quarantined by AVG.

I think the commercial AVG system adds anti-spyware and maybe firewall functionality, but I might be wrong!