Microsoft grants a 30 days test period for the new Windows version. For this trial a product or activation code is not required. But the duration of this test can be extended to 120 days with a Windows internal mechanism.

To achieve this you have to run the cmd.exe, which can be found by typing "cmd” into the Windows search, as an administrator. In the now open command prompt you have to type: "slmgr.vbs -rearm”.

This sets the activation time of Windows back to 30 days after the system is restarted. All in all this trick can be used three times, extending the trial period to a maximum of 120 days.

This trick works for XP and Vista too, says Tweakpc.