Most PC's purchased come with either Symantec's or McAfee's security suites installed with a 90 day trial period. For over to years I've been using Bitdefender's anti-virus software and recently upgraded to their
Total Security Suite.
BitDefender Total Security 2008 - 2 Years/3 Pc's
I use Bitdefender because it seems to place lower overheard on my system and I've never had an application conflict. The new suite has additional functionality with anti-phishing, anti-spam, anti-spyware, a registry cleaner, and parental controls. In addition, it has a backup tool. All options are selectable and easy to configure and it works well with my Vista system. If you already have paid for a subscription to an anti-virus or security software, Norton, McAfee and many of the others do a good job as long as you keep your virus definitions up-to-date, but if you are looking for a new one, I'd recommend Bitdefender.