Extending The Windows Vista Grace Period to 120 Days

Every Vista DVD includes the ability to install any edition of Vista without a product key. When you install without a product key, you get an automatic 30 day evaluation period.

You can easily extend the 30-day Windows Vista grace period to 120 days. No hacks required. This is an official, supported operation directly from Microsoft. To extend the grace period another 30 days, simply start a command prompt as Administrator, and issue this command:

slmgr -rearm

Reboot for the change to take effect and you have 30 more days. You can only extend three times, so the total grace period for a Vista evaluation is 120 days. ( source )