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 AV/Malware solution that's looking to eradicate infections is MSE, they are from Microsoft, and MS doesn't want our systems infected, hence the birth of MSE, totally free. The only cost incurred is having a genuine copy of Windows.
The paid ones doesn't give a damn if the net is full of infections, as long as they can "contain" them, or "quarantine" them, as the term is called to us. Honestly, they don't. It's their bread & butter, their livelihood. As long as the users pays them their yearly membership fee, they'll continue to be protected (I hope). And that brings in another point altogether, no AV can be correct 100% of the time, under all conditions. There's no way. If any single AV/Malware suite could guarantee this, and deliver, that one company would be wealthy, because we all would turn to it. But it's fantasy to even think that a single solution can keep up with all of the virus samples that's produced daily.
So to answer the question as to free vs paid AV's goes, I say free, if either you choose MSE (the only one that's even trying, not making any money on it), or a good free one along with MBAM as a standalone scanner as second string protection (we need a second string plan anyway).
And remember, the best AV is the one between the chair & keyboard. Practice safe computing practices, such as not opening "spam", not clicking onto links that's too good to be true, and scan each and every attachment that you get, no matter what the source. Your friend can be carrying a virus, and not know it.
Safe & Happy computing to everyone,
Cat