Vista includes Windows Defender, which is antispyware software. Unfortunately, Windows Defender doesn’t protect you from viruses, worms, and Trojan horses, which are the nastiest types of malicious software. They’re the ones that cause all the damage and can really make your computer unusable.
Antivirus software designed for Windows XP won’t work on Windows Vista. Currently, all the antivirus companies (including Microsoft OneCare) are updating their software to run on Vista, and most of them have beta versions available. For more information about antivirus software, check CNet.