High Quality Content by WIKIPEDIA articles! System Locked Pre-installation, often abbreviated as SLP, is a procedure used by major OEM computer manufacturers in order to pre-activate Microsoft Windows before mass distribution. There are three different versions of SLP: SLP (which is now commonly referred to as SLP 1.0) SLP 2.0 and SLP 2.1. These versions roughly coincide with Windows NT versions (see table below). 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 user is prompted to activate his or her copy as normal. SLP 2.0 and SLP 2.1 works 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 Windows copies.