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. This probably isn't news to anyone.
What may be news to you, however, is that 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 voila, you have 30 more days. You can only extend three times, so the total grace period for a Vista evaluation is 120 days. Source: Pctorium.com