Let me correct some things here before people jump on board with ClamWin.
ClamWin is not sufficient to be used as an exclusive AV on any system. I believe last time I talked to ClamWin people, they had about 28,000 Virus/Trojans in their database. To let you know how inadequate this is, the average commercial antivirus has roughly 90,000-120,000 definitions.
Second, ClamWin doesn't utilize a realtime monitor, so its essentially useless at stopping infections BEFORE they happen, which is critical, because once the damage is done, who cares if an AV finds it? Its too late.
Third, its heuristics are extremely lacking. Without strong definitions, and product has to rely on Heuristics, but if neither are up to standards of modern commercial applications, then the product fails - most of the time.
Fourth, updating an antivirus is a time consuming and extremely busy task. Most commercial AV's have labs with 50-150 people working in them handling virus/trojan analysis, and signature updates, in addition to coding. A freeware AV will never, ever, be as competant as a commercial one because of this. This includes Avast, AVG, and all of the other freebie ones, they have substandard detections.
ClamWin, on the last wide scoring I saw, tested in the 30% range on threats. Considering most AV products score in the 80-99% range, this is pretty lousy. Now you might ask, what do I recommend? Well, I own licenses to every AV product on earth, and have done virus analysis for about a year for 4 different AV companies. I have three products I recommend, depending on your machine, and circumstances.
High risk, Light Gaming or otherwise general computer: Kaspersky AV
Medium Risk, General Computing system, Medium Gaming: NOD32
Medium/High Risk, High Gaming Machine, General Computing: Arcabit
The reason is simple. Arcabit has zero impact on system performance and gaming. There isn't a game on the market that it will crash. With great signatures and heuristics, its more than suitable for gaming rigs under medium/high security risks.. Kaspersky has the best definitions in the world, and easily the best detections, but can be a slight load on systems, and can interfere with many games. NOD32 is nice, and semi-light, but has been known to interfer with some games as well.
My 2 cents.