If you own a legal copy of Windows XP, you wasted hundreds of dollars to buy Windows Vista upgrade version and if you wanted to perform a clean install of Vista, then you are out of luck. According to Microsoft, you can use the upgrade version only to upgrade over Windows XP. You cannot use it to perform a clean install (which you are allowed to in XP). You can either install it over a clean install of XP and install it over your full of crap XP or waste hundreds of dollars and buy a regular version. This is to stop the piracy (?).
Best option: Move to Linux.