The new Vista operating system is very expensive, but luckily offers an upgrade version for those who already have Windows XP. The only problem is, to use the upgrade, you must upgrade over an existing XP installation, which then leaves you with a bunch of old junk on your computer. Another problem is that Microsoft tries to invalidate your XP key as you upgrade. Thats not cool, especially if you decide you don�t like Vista and want to switch back. Here is how you can install a clean installation of Vista from an upgrade disc.
Basically it involves installing a trial version of Vista, and then upgrading from this to your full version. Because you are upgrading from the same OS, your key remains valid, and your trial files are just overwritten, and don�t affect your system at all. Of course, you should only do this if you have a version of XP and are legitimately upgrading.
Full Instructions [WindowsITPro]