All versions of Vista allow a 30-day period without activation, except the corporate-oriented Vista Enterprise, which supports only a three-day trial.
But there is a way to extend the trial period from 30 days to as much as 120 days. A very simple way.
Just start a new command prompt and type:
Reboot your pc and you will have 30 more days to play with your Vista. However, you can extend the grace period only 3 times, which means you can use Vista for upto 120 days without activation.
If you are wondering if the above method is legal, stop worrying. Using the above command to extend the grace period is not a violation of the Vista End User License Agreement (EULA). This has been confirmed by a Microsoft spokeswoman.