Plain and simple. A upgrade is dodgy at best. If you use XP for a period of time before you get Vista and then upgrade from XP to Vista if there is ANYTHING wrong with the XP install, spyware, virus, file updated that shouldnt be, anything that has been messed with can corrupt your upgrade and cause you grief and not make your system run right.
It is always advised if you can install fresh to do so. Upgrading is a tricky business.