Now Microsoft has confirmed that Windows Vista can be used and run for 120 days or 4 months (3 rearms), and extending the activation grace period is not a violation of the Vista End User License Agreement (EULA). All versions of Vista include Windows Vista Ultimate allow a 30-day free period without activation, except the corporate-oriented Vista Enterprise, which supports only a three-day trial.

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

1. Start command prompt with administrator privileges(run as administrator)
2. In the Command Prompt, type the following command

slmgr -rearm

or you can use sysprep /generalize

3. Reboot the computer.

4. Rearm again when the remaining activation grace period timer counts down to 0 days.

Rearm option resets the computer’s activation timer and reinitializes some activation parameters.