Some of the more budget-friendly systems come with a free Linux operating system to make them more affordable. Adding this option gets a copy of Windows XP installed on a PC that doesn't already have it.
ALSO WORTH MENTIONING... is the fact that when you have Windows added, you get the genuine installation media and product key that used for your PC, in the event that you should need to reload your hard drive.
NOTE: This option is only available for combined purchase with a new PC. No copies of Windows software will be sold separately.