Your operating system (OS) is the software that operates your computer's resources and programs. One often comes pre-loaded onto your computer when you purchase it. The most common operating systems are Microsoft Windows and Apple's Macintosh (Mac) OS X. Operating systems have specific versions. For example, a Windows user might be running Windows XP or Windows Vista. Mac users should be able to check the version of their operating system by going to the Apple menu (in the top left corner) and selecting "About This Mac." Windows users can check the version of their operating system using steps provided on the Microsoft website.