Computer manufacturers such as HP, Compaq (who are now a part of HP), and Dell, set up on some of their hard drives something called a Recovery Partition, which contains all the elements of a fully installed version of Windows and whatever other software came bundled with the package. There is a specific keystroke combination that you press on bootup in order to initiate a Recovery, which consists of the options to repair, reinstall, or access the Recovery Console. The Recovery Partition represents an alternative medium for your Windows volume and OEM bundled software. As it comes as a preconfigured image, it saves the end users (the majority of whom don't entirely know what they are doing, which is almost a necessity these days) the hassle of trying to actually install and configure the OS. The Windows versions on these systems (almost invariably XP home) come pre-activated. But this medium replaces bundled CD based software. Mind you, if you have one of these systems, you have the option to order a CD version of this software bundle.
If you want to know your CD-key, you can download a program called AIDA32,
which will provide a huge database of information regarding your PC configuration. Go to the Operating Systems section for your CD-Key, and much other useful information.