What is OEM software?
It stands for Original Equipment Manufacturer. In terms of Windows it refers to cheaper versions of its products with simple packaging, and reduced support.
Can i buy it?
Most users of OEM software on eBay fall under the last category of those that install software.
"Local OEM software packs are intended for PC and server manufacturers or assemblers ONLY. They are not intended for distribution to end users unless the end users are acting as local OEMs by assembling their own PC. " http://oem.microsoft.com/downloads/mosb_productguide/licensing/packaging.htm
This software is not intended for end users, the 'Joe Newbie' who does not know how to install software and needs the support the retail version provides.
Microsoft says that people who purchase OEM software will simply be expected to abide by the terms of the licensing agreements.
So why is it so cheap?
OEM software can't be returned once opened. So if you bought it and need to return it, don't open it.
Also the software is tied to the motherboard it is first installed on. While retail versions can be transfered from computer to computer, OEM versions are non-transferable. You can upgrade any other hardware you like but must keep the same motherboard.
While some retail versions (eg Vista) come with both the 32 bit and the 64 bit versions, OEM versions are specific.
Lastly there's support. As a system builder purchasing OEM you are your own support. Whenever the end user has a question they will come to you. Retail versions normally come with 30-90 days free support. There is also a paid support option from Microsoft if you need it but it is quite dear.
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Just a quick mention that the column on the right hand side under the title 'Items from eBay Sellers' are NOT my items, just ones that eBay thinks as relevant. Most likely they are advertising Open Office. Don't fall for the trick! Open Office is not they same as Microsoft Office, and furthermore you can get the latest version for free at openoffice.org. It's a shame that those sellers don't mention that important fact.