You will need
  • program AIDA64 Extreme Edition;
  • program TuneUp Utilities.
The Internet to find the program AIDA64 Extreme Edition. The app is a paid app but has a free period of usage which is one month. Install the software on your computer. Launch it and start scanning your system. After its completion, will open the application main menu.
In the right window menu, click on the option "Operating system", in the next window, select "Operating system". A window will open which will have very detailed information about your operating system. Data will be available in a few sections. Find the section called "License information". In it find the line "product ID". The number in this line is the license number of your version of Windows.
Another program with which you can know the number of the license, called TuneUp Utilities. Download it from the Internet. In this case you must consider the version of your operating system like Windows XP may not be compatible with Windows 7. Install the program on your computer. Start it.
After the first run of TuneUp Utilities need to wait a bit because there will be a full computer scan. Then you will be prompted to correct errors. If you agree, the program optimizes Windows, then you will be taken to the main menu of the application. Or you can give up, in this case, the menu appears immediately.
Select "Fix problems", then "Show system information". Then go to the tab Windows. In the opened window look for the section "License", and in it the string "program ID". Opposite this line is written the serial number of your operating system. You can also see other information on license Windows.