Windows Vista: The Vista Welcome Center gives you pertinent information about your operating system at the top of the Welcome window. If the Welcome Center is not on your screen when you start the computer, you can access it by clicking the Welcome Center icon in the Control Panel. (Choose Start, then Control Panel to access the Control Panel. 

Macs: All Apple computers use the Mac Operating System. To determine which version you are using, just click on the Apple menu and choose About This Computer.

Windows 95/98/Me: Hold down the ALT key and double-click on My Computer. The System Properties window appears. The name and version of the operating system will be listed in the System area of that window.