This seems to differ slightly between different versions of Windows. The following works for Windows 7 Professional (Version 6.1, Build 7601: Service Pack 1).

Click the Windows icon start menu button (usually at the lower left) which may look like this:

Click on ‘Control Panel’.

Click on ‘Ease of Access’

Click on ‘Ease of Access Center’

Click on ‘Change how your mouse works’

Under the ‘Make it easier to manage windows’ heading, tick the box next to the option that says ‘Prevent windows from being automatically arranged when moved to the edge of the screen’

Click ‘OK’

Click the Start button or Windows icon. Usually this is in the bottom right corner. It may not say ‘Start’; instead, it might look something like this:

In the search box where you can freely enter text (which may or may not say ‘Search programs and files’ in light grey letters), type:

winver

and a pop-up window should appear telling you the version of windows.