Windows Vista doesn't have anti-virus feature built-in and you have to use the 3rd party product to have this protection equipped. For anyone who uses Symantec AntiVirus Solution to protect your computer from being attacked by virus, you might get the incompatible warning message while you install it on your Vista machine. To keep your pace up, Symantec just released a new version for it, which is version 10.2. However, you have to have the certification code to login to get it downloaded. For more information about the Symantec Antivirus for Vista, check the following link.
Symantec AntiVirus Corporate Edition for Vista