Windows Vista upgrade version does not allow clean installation.
New versions of the Upgrade version of Vista doesn’t allow new installations…
What does all of this mean on a practical level? Users who purchase upgrade copies of the aforementioned versions of Vista will find that they can only upgrade PCs that already have Windows installed. KB930985 clearly states: “you cannot use an upgrade key to perform a clean installation of Windows Vista.” According to Microsoft, this happens because Windows Vista does not check for upgrade compliance. If you do not have a previous installation of Windows available, Microsoft recommends that you “purchase a license that lets you perform a clean installation of Windows Vista.” And what does it mean to your wallet? More $$$ out of your pocket if you want maximum flexibility.
More info on this bone-headed move here or here
I have a lot more information, a full review, and recommend which version, for more coverage of my experiences with Windows Vista, problems reports and my recommendations, go to http://pstech.wikispaces.com/Windows+Vista
To check if you hardware software has compatibility problems, run the microsoft Vista upgrade advisor and check this list for hardware and this list for software, since Microsoft has decided not to publish a list of software apps that don’t work.