originally vista designed to have its own anti virus along with anti spyware and fire wall and many other system performance tune up tools

but as far as i know, Microsoft changed their mind and made this package for sell (also to xp users, here is a link http://www.windowsonecare.com/) ans so vista users who are'nt willing to buy it will probably have the same free defense programs (like the simple built-in firewall and the free windows defender)

here is another link to an article about the vista's anti virus issue