I prefer ClamAV myself, but if someone is looking for a free antivirus solution, Microsoft has proposed an answer:

Microsoft plans to introduce free security software for personal computers and discontinue the subscription-based Windows Live OneCare service. The new security suite, codenamed Morro, will be available in the second half of 2009 and will protect computers against viruses, spyware, malware and Trojans.

The cost of antivirus should be as integrated into the OS as the cost of brakes is integrated into a car.