The answer to this question is very complicated, since no single app can do everything needed to keep your computer relatively safe. Ultimately, the job of keeping your computer safe is YOURS, not a piece of software.
As to whether a free product is better (or worse) than a commercial product, well, the old saw, "You get what you pay for." is still true.
1) NOD32 Antivirus (latest version, with latest A/V signatures <http://www.eset.com> I use the XP/Vista firewall with NOD32, along with IE 7 and its built-in "phishing" and "Popup" controls. NOD32 also looks for and removes Trojans, dialers, root-kits, and other forms of Malware, including unreported ones, using advanced heuristics. I've never been infected while NOD32 was installed. In addition, NOD32 A/V is the FASTEST scanner on the market.
2) Trend Micro Internet Security (PC-Cillin Security Suite 2007), with latest A/V signatures <http://www.trendmicro.com> PC-Cillin is pretty-much the best "suite" on the market. Like NOD32, it looks for root-kits, "phishing", Popups, and other forms of malware, in addition to being the second-best A/V product. It's also a great way to save money, and still get excellent protection. TrendMicro was the first company with a Vista-compatible product available to users. However, it has a SLOW scanner.
3) Kaspersky Internet Security 6.x, with latest A/V signatures. Like the two above, it looks for and removes not just "traditional" viruses, but also other forms of Malware, using advanced heuristics. Many say that the Kaspersky A/V engine is the best on the market. But I say that Kaspersky, while a great A/V product, uses too many Windows resources, and slows the OS down considerably during scanning. In addition, its download servers are NOT the kind one wants to use with their Broadband connections, since the FASTEST download speed I've seen using Kaspersky and my 12Mips cable connection was in the range of 3KB/sec-20KB/sec. I certainly wouldn't put it on a slow machine, or one with dial-up access to the Internet.
1) Avast A/V Home Edition, from http://www.alwil.com This one is excellent for the price. It also has a very interesting GUI, and most warnings, etc are vocal. Updates for the A/V signatures are not often enough for me, however. The Pro Edition (commercial) has "push" updates as often as you want them. That's pretty much the only difference between the Home Edition (free) and the Pro Edition (costs). The free product is updated about every three days. Avast A/V (both the free and commercial product) has a beautiful GUI. But while its GUI is great, its efficiacy as a good A/V product is not so great, having one of the lowest scores at finding and removing viruses and other malware. But it is probably the "best" free A/V product.
2) AVG A/V Home Edition (the latest version, with the latest A/V signatures) is a fair-to-middling A/V produt. It seems to be the most popular of all the "free" products. But it just does not measure up as well as it could, or should, having a very low score when it comes to finding and removing malware.
I can't think of another "free" product which is efficious at it's job of finding and removing malware, so there is no third product in the list.
Any of the three commercial products above are relatively "trustworthy", and do a good job of protecting the user from attacks which are out of his control. Knowing my aversion to "free" products, I cannot say with any certainty that either of the two listed "free" products are "trustworthy".
I can say with certainty that I've never gotten infected using ANY of the 5 products listed above. But I use OTHER methods to help keep my computer clean.
Which brings me to the MOST IMPORTANT methods for keeping malware-free:
1) NEVER click on links in E-mail. This is the NUMBER ONE way viruses get onto computers. Which is why so many hackers use "social engineering" in e-mail messages to get their malware on unsuspecting user's HD's.
2) STAY AWAY from Internet Porn sites. PERIOD!!!
3) STAY AWAY from P2P programs!!! Get your music and software the HONEST way, and BUY it, and you will be kept from a GREAT DEAL of malware. P2P programs are probably the SECOND-MOST dangerous programs, after e-mail clients, that hackers use to insidiously take over machines.
4) Do your BEST to STAY AWAY from websites which are based BEHIND the former "Iron Curtain". While the best A/V products are produced by programmers and companies which are based in the former Soviet Union, the best hackers are also based there.
5) ALWAYS make sure your Browser's "Phishing" controls are TURNED ON when entering unknown territory.
6) KEEP your computer education UP-TO-DATE!!! Many older viruses and other forms of malware are STILL taking a hugh toll on ignorant and misinformed users, when they should have all fallen by the wayside by now, and consigned to the bone-yard of useless malware.
As you can see, the individual user is the MOST-IMPORTANT link in his anti-malware "fence". NEVER depend on others to do what only you can do for yourself. Ultimately, NO anti-malware product can do what only you can do.
And NEVER let your EGO get in the way of proper security measures. Personally, I would rather be safe than sorry. In the end, YOU are the ONLY ONE you can trust.