PC Pro ran a massive anti-virus labs test this month, focussing purely on AV/malware solutions, not these kind of "complete security packages" that do firewalls and phishing detection and all that lot.
They put Kaspersky down as absolutely the clear winner by some distance, and also commended it for a very small memory/CPU footprint. Only downer is of course that it's not free! (It had a 98% detection rate.)
If you're after a free AV solution, your best bet is Avira AntiVir Personal Edition Classic, which managed a 92% detection rate but apparently nags quite a lot for you to upgrade to the premium version.
AVG Free Edition only managed an 83% detection rate, and whilst it was praised in some areas, wasn't considered effective enough at detecting threats to be strongly recommended.
The other freebie is Avast 4 Home Edition (a feeble 70% detection rate), which was given a bit of a pummelling and not recommended at all.
Norton and McAfee both did poorly, with 68% and 78% detection rates respectively - Norton's performance in particular was very bad, and was also a resource hog when running under Vista.
Special mention was made of Microsoft's own Windows Live OneCare which managed an appalling 66% detection rate and also took it upon itself to alter system settings as it saw fit, it got one star out of six.