OEM versions are versions meant for people who sell whole computers.  It's the version of software that would come preinstalled on a computer.  It cannot be used to upgrade from a previous version of the software.  It won't work if there is a previous version of the software on the computer, as it's been to be installed on a hard drive that didn't have any software on it before.  OEM software does not come with manuals nor any technical support other than what is available at the manufacturer's website.  OEM software CAN be upgraded to a full retail version, in fact, it's pretty much expected that you do evenutally upgrade to a full retailv version.  Finally, NO SOFTWARE VENDOR TAKES OEM SOFTWARE BACK. and of course that includes us.  People try to buy OEM software and get the key code, because usually you can see it without openening the software and installing it, and returning it.  It's a scam, therefore not software reseller takes OEM software back, so be sure you understand how to use OEM software before you purchase it.  Otherwise, it's a fully functioning version of the software.