It is a serious risk to run without an anti-virus.
Also, free versions are generally, not as effective, in any way, as paid anti-virus. I always recommend ESET Smart Security for a reason. There have been links posted by spammers on this site that have sent people off to fake versions of software like VLC Player. In these instances Microsoft Security Essentials could not detect that the executable was virus-laden. ESET Smart Security picked it up almost immediately, and it has picked up viruses in virtual machines on me in the past. I would suggest going the paid route.
The BIG Problem
There is a common and widely held belief that computers which are unused but left on require little to no anti-virus or maintenance whatsoever. This has resulted in an army of millions of computers around the world twisted into "botnets". These systems have been completely compromised and traverse the Internet sending out spam, junk, and other garbage. Many of the users of these systems don't even know this is going on.
Better to go with pro-active, paid protection. And if you can't afford it, look up:
AV-Comparatives - Independent Tests of Anti-Virus Software - Welcome to AV-Comparatives.org
Magazines are often paid to write editorials and reviews on anti-virus products, and many of the "big" ones, i.e. Symantec likes to take out enormous full page ads to explain how great their software is while computers are being laid to waste. It has always been my experience that anti-virus solutions like Symantec and McAfee have been hot on marketing and cold on heuristics and definitions.
When we talk about anti-virus software, we no longer deal in just viruses, but in the ability of the software suite to do what I would consider the following:
1. Identify and quarantine viruses, spyware, malware, trojans, rootkits, adware, and other non-virus potential threats.
2. Prevent unauthorized network access using pro-active heuristic scanning on incoming and outgoing traffic.
3. Provide the end-user with a means of accessing threat assessments and data related to possible threats.
4. Do all of the above without turning the entire computer into "Mr. Norton's Extremely Slow Computer"
With Symantec, we are dealing with a software company that had to change the name of its software because it had such a bad reputation of completely taking over the computer and slowing it to a complete crawl. While that problem has been eliminated now in End Point and later versions, it still embeds itself so deeply in the operating system, I cannot sanction it myself.
Personally, the only two anti-virus products I trust, commercially, and know for a fact that they do a better job from experience are ESET Smart Security, NOD32, and Kaspersky. I used to swear by NOD32, but after deploying their Smart Security suite, I would not go back to the simple anti-virus.
These days you need to be covered for potential threats. The biggest problem is phishing attacks (fake websites that purport to be something they are not). In the area of these products, we have heuristics and definitions.
Definitions are programmed by the anti-virus manufacturer with instructions about threats and how the program can detect and eliminate them. These definition files are sent as updated to the anti-virus.
Heuristics determine how well the software is at identifying and mitigating these threats "in the wild", and with existing definitions, so even if a new unknown threat exhibits behavior that would damage your computer, it will prevent that attack. So if an unknown virus came out of nowhere, we have heuristics to back it up if definitions aren't available. Of course heuristics are also used to interpret definitions.
To understand why anti-virus products are needed, we need to go back to the Blaster Worm debacle as well as the Code Red issue. Look up both of these and see the damage they caused. After this, it seems like free anti-viruses became the norm, and even Microsoft published their own free anti-virus.
I would not say Microsoft Security Essentials is a bad product. For the price (0), it is excellent. The same can be said for AVG and Avast. However, for a real comparison of anti-viruses, you must use an independent lab to find out how these products have performed.
Link Removed due to 404 Error gives you that information. You will not find it in PC Magazine or some other place where paid advertising is the order of the day.
From my experience, NOD32 (and thereby ESET Smart Security), has been the most consistent security solution that does not eat up enormous resources. But many of you will have different experiences. If I did not have the money for ESET Smart Security I would use Microsoft Security Essentials and Firefox or Chrome for added security.
So check out
AV-Comparatives and
AV-Test for this information.
The landscape is always changing. A few years ago, BitDefender seemed like the best one ever, and then it just dropped off the radar with people getting hit. This was mostly with XP systems, and you certainly are more secure using Windows 7. I simply cannot fathom the idea of using no anti-virus at all. This would be a major mistake. And by all means, consider a paid, full security suite to protect your hardware/software investment. If you think about it, this is what you are doing. Not to mention protecting, maybe, years of work as well.