System Locked Pre-installation, often abbreviated as SLP, also referred to as OEM Activation, is a procedure used by major OEM computer-manufacturers in order to pre-activate Microsoft Windows before mass distribution. Details SLP comes in eight different version which roughly coincide with versions of Windows NT. Operating systems that use SLP 1.0 check for a particular text-string in a computer's BIOS upon booting. If the text string does not match the information stored in the particular installation's OEM BIOS files, the system prompts the user to activate their copy as normal. SLP 2.0 to SLP 2.5 work in a similar manner. This effectively "locks" the operating system to the qualified motherboard. In addition, if an end-user feels the need to perform a "clean install" of Windows and if the manufacturer supplies the user with an installation disc (not a "System Recovery" disc that is a hard-drive image), the user will not be prompted to activate the copy, given that the installation is performed on the same motherboard. Furthermore, because the check only involves the BIOS and not hardware, a user is allowed to change virtually all hardware components within the machine except motherboard, a procedure that would normally trigger re-activation in retail copies of Windows. If the replacement board is the same brand and of the same broad vintage as the original, it will often be accepted by the OEM software. Manufacturers are generally only given a few SLP keys by Microsoft. A royalty has been paid by the manufacturer if the board has an SLP key. Each manufacturer is issued with their own modified copy of Windows XP or Windows 7 that recognizes their own unique keys. Windows 8 will install on any UEFI motherboard containing a valid encrypted Microsoft product key since a royalty has been prepaid. It is most probable that an individual OEM royalty is much less than the retail price of the same version of Windows. The product key sticker attached to OEM computers, prior to Windows 8, will not activate the copy of windows supplied with the machine. Only the SLP key can do that and the product key is never requested during activation. The purpose of the external sticker with a key is not known with certainty, but it is most likely a way for Microsoft to check that items for retail sale have had a full royalty paid by the OEM. SLP installations