First, there is no version of Windows, including Vista, that comes with its own built-in antivirus. Vista does have its own built-in firewall. Vista also includes an anti-spyware product, Windows Defender. Neither the firewall nor Windows Defender have antivirus components.
Most new computers come with a 60 or 90 day trial version of an antivirus. However, once the trial period has expired, one must either purchase a subscription to the antivirus product, or obtain a new antivirus product.
If you prefer a free antivirus, then AVG, avast! or AntiVir have free-for-personal-use versions available, all of which are suitable for Vista.