Retail:
This full version of Vista has no limitations. Retail can perform clean and upgrade installs. Retail can be transferred to upgraded or entirely new systems as long as only one copy is installed at a time. Retail also includes full support from Microsoft and is the most expensive edition.
Upgrade:
This limited version of Vista can only perform upgrade installs on existing installations of qualifying Windows products. However, there appears to be a work-around for Upgrade to perform clean installs, like Retail and OEM versions. Upgrade may be transferred to an upgraded or entirely new system as long as only one copy is installed a time. Upgrade includes full support from Microsoft and is mid-priced.
OEM:
This limited version of Vista can only perform a clean install and cannot upgrade. The EULA has been changed from XP OEM and Vista OEM now states that it can only be installed on one computer and is forever bound to that computer (motherboard). This has raised eyebrows, but various tech sources online have indicated that Microsoft's product activation center is not currently enforcing this rule, but only showing concern for whether or not your copy of OEM Vista is installed on 1 or more computers. This rule in the EULA may only be a technicality to discourage people from buying less-expensive OEM versions instead of Upgrade or Retail. However, even if MS gives you an activation key for your upgraded or new system, you are still technically violating the EULA and MS may choose to take action in the future. OEM is intended for system builders you must provide your own support. OEM is the least expensive edition.
