If you choose carefully, some free products are as good as their paid counterparts. For years, I used Avast (free home version) on XP Pro, and it kept the nasty bugs out. Once in a while, I'd do a scan with MBAM or Windows Live Safety Scanner (both also free), just to make sure that Avast was doing it's job.
Today, I'm running Windows 7 Pro (x64), and MSE gives me all of the protection that I need, is lightweight, no BSOD's, you can update it through MS update. I keep tabs on it by an occasional scan with MBAM & ESET Online Scanner.
The problem with the paid AV's is this: They seek to contain only, so that they can get you to renew over & over. The AV industry loves viruses, this is how they make their money. Possibly the only...