Software suppliers generally offer various promotions to encourage users to move over to their security application and, once installed, as long as the user doesn't experience a problem, it's unlikely they'll bother to switch to alternative security software. For this reason, look around and you'll find various long-term security licenses available for you to test the software for longer than the standard 30-days.

CA AntiVirus is a commercial-grade antivirus tool but still worth examining for the average home user. We've found an extended 12 month license which is open to everyone, but doesn't appear to have an end date, so we'd advise you sign up sooner rather than later.

CA AntiVirus link. 

13 Jun 2008