The manual and the set up screens did not make this clear to me when I made the mistake of buying and installing Vista two years ago:
# Upgrading a 32-bit operating system to a 64-bit operating system If you purchase a 64-bit upgrade version of Windows Vista with the intention of installing on a computer that is running a 32-bit operating system, you must install Windows Vista by starting the computer from the 64-bit Vista installation DVD.
Important Do not remove the previous operating system before you start the Windows Vista installation. This is because Windows Vista Setup has to verify that your current operating system license supports the installation of a 64-bit upgrade version.
# “N” versions of Windows Vista The “N” versions of Windows XP and of Windows Vista do not include Windows Media Center. You cannot perform an upgrade installation of an “N” version of Windows. Instead, you must perform a custom installation.
Bold italics mine. Without these explicit instructions, one gets the impression that they cannot install the upgrde with any other choice than the 32 bit upgrade process. This note is buried in the middle of KB932795.Dang, the one thing Microsoft was originally really good at was documentation. Instead, I have spent two years running a 32 bit OS on a 64 bit machine.