Downgrading Windows Vista Back To Windows XP
 

In this PCSTATS Beginners Guide we cover the many steps necessary to either reverse an unfortunate upgrade to Windows Vista, or to re-install Windows XP on a brand new PC that came pre-installed with Vista. Sharpened wooden stakes, garlic and holy water are recommended, but not essential. First the unfortunate truth: There is no 'undo' button for getting rid of Windows Vista. Sadly, there is no easy way to go back to Windows XP as such... you'll either have to remove the entire operating system and start fresh with Windows XP, or do nothing at all.

http://www.pcstats.com/articleview.cfm?articleID=2311