I currently run Avast a/v, and Online Armor (both free versions) on my XP p.c. I also scan weekly for malware.
I use the Comodo firewall (free version) on my XP lappy, with no a/v. Again, I scan weekly.
The only problem I ever had was having my e-mail "spoofed", but that was after someone else posted it on a public website. Thank you very much.
My son uses the Comodo suite (free version) on his Vista lappy. As far as I'm aware he hasn't had any problems, though I haven't checked it recently. My other son uses Norton that came with his Win7 lappy. No problems there, either.
I've tried most of the free security softwares (I'll try anything on this lappy!), and the biggest problems I've encountered are failed updates/upgrades and resource hogging. The difference with paid-for software is that you should get full technical support if something does go wrong - obviously using free software I don't expect any!
I believe the best security is just being very careful how we use the 'net.
Just my opinion, and not something I want to get into a debate about!
Best o' luck, Gooddog.
'Hat.