Windows Vista can be installed, and run without any license, product key or activation for 30 days, This is solely for the purpose of trial/evaluation. Microsoft stress's that users should purchase a license with a valid product key before the trial period expires, or else Windows Vista will lock into Reduced Functionality Mode.

Although, A method has been discovered which will allow you to extend the evaluation period for a fresh 30 days, 3 seperate times. Microsoft has confirmed that extending the evaluation period is not a violation of the EULA and is aware of this tweak but does not seem to mind one bit, as no fixes or updates have been released to fix it.

All versions of Vista include Windows Vista Ultimate allow a 30-day free period without activation, except the corporate-oriented Vista Enterprise, in which case it supports only a three-day trial.

To reset or restart the initial grace period of Windows Vista to another 30 days, use the following steps:

1. Click on Vista Start button and type in "Cmd" in Start Search box (without quotes).
2. Press on Ctrl-Shift-Enter to open Command Prompt with administrative credentials (equivalent to “Run as Administrator”).
3. In the Command Prompt window which you opened, type the following command and press Enter when done: "slmgr -rearm" or "sysprep /generalize" (without quotes)
4. Reboot the computer.
5. Run the script again when the remaining activation grace period timer counts down to 0 days, you can run the code sooner but it is a waste. As if you have 20 days left and you run the script it will bump you back up to 30 again not 50.