Is software intended for installing on newly built PCs. Software companies sell in bulk at greatly reduced prices, but don't provide support, which should be provided by the PC vendor.
From About.com click here
"OEM software is only distributed when sold with specified accompanying hardware. When these programs are copied and/or sold separately from the hardware, it is a violation of the license with the software publisher, and therefore illegal."
Much of the OEM software that you find on the web, or via email advertising, may well be counterfeited anyway, so you will be paying for nothing.