If you’re not confused enough by all the versions of Vista, the upgrade choices might just do it. Do you want the full-retail version, the upgrade version, or the OEM version? The obvious difference is price. Most places are selling the full retail version for about $230, the upgrade for about $150 and OEM for $120. So, just what is the difference?

Full Versions can be installed on any system, whether it’s a new system you just built with no operating system, or a system with any previous version of Windows installed, or any other operating system. You can also upgrade or change that system and reactive Windows as many times as you would like. Includes both 32 and 64 bit versions (to be installed on one PC).

Upgrade Versions must have Windows XP already installed on your hard drive of the system that you plan to install the upgrade on. Windows 2000 is also eligible for the upgrade (although a clean install must be performed), earlier version are not. Windows XP only required that you possessed the media from an earlier version (a CD for example). The new requirement with Vista is that the earlier operating system must be installed on the hard drive you’re upgrading. After Vista is activated, the XP key that was upgraded becomes invalid. You can’t use that key to install XP on another system, or in a dual-boot configuration. For example, say you purchased the upgrade edition to install on a new system. You must first install and activate XP, and then use the Vista DVD to upgrade. After which you can’t use the XP key again. Both 32 and 64-bit versions are included (to be used on one system). Link to: Upgrade Matrix

OEM Versions are intended to be sold to Original Equipment Manufacturers. There is no phone support from Miscrosoft provided, no retail packaging, and no manual provided. Support is supposed to be come from the system builder. An OEM version will allow either an upgrade or a clean install. You must chose 32 or 64-bit verisons, they are not both included like they are with the retail vesions. Finally, OEM licenses can’t be transfered to another system. You can’t transfer from an old computer to a new one. If you upgrade your system, you’re limited as to what you can change, and how many activations you’re allowed.