There can be different situations when you need to know your Windows version. For example you want to install updates, or perform repair install or upgrade your product.  And it can takes a lot of time to explore this until you know following solutions.

1. The easiest way is to use built in windows application. Winver.exe. Press Start->Run and type there: winver.exe and press Enter.

You will get following window with all required information:

2. Right click on My Computer:

3. Go to C:\WINDOWS\system32\ and open prodspec.ini

4. In Command shell type:

reg query "HKLM\Software\Microsoft\Windows NT\CurrentVersion\" /v BuildLab