From Arstechnica.com:

"Upgrade versions of Windows Vista Home Basic, Premium, and Starter Edition will not install on any PC unless Windows XP or Windows 2000 is already on the machine in question. In years previous, upgrade versions of Windows could be installed on any PC. If a PC did not have an older version of Windows installed, users could provide an older installation CD of Windows for verification. After dropping a qualifying CD in the CD-ROM drive, the installation routine would verify the disc and you'd be on your way. With this approach, one could use an "upgrade" copy of Windows to lay a new Windows install on a computer."

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