If you were one of the many who were more than slightly miffed at Vista's ridiculous upgrade requirements, you are in luck. By default, Vista now requires you to upgrade from an existing install, as well as forfeit your former Windows XP license. This is different from 2000 or XP, which will allow you to install from an upgrade disc if you merely provide the media from an older version of Windows during the install. While the apparent theft of the license is in Microsoft's hands alone, there is at least a suitable workaround for being able to install fresh using an upgrade disc.
The solution lies within Vista's ability to let you install it as a 30 day trial, from the upgrade DVD. You can simply skip entering your CD key, then re-running the installer after it is finished and entering your key then. Unfortunately, it means installing Vista twice in a row. You don't save time, but it may save you a few angry phone calls or a trip to the store if you end up having to do a complete reinstall. MS has still not commented on why they implemented these new, easily frustrating, install "features".