Is the standard 30 day trial period or initial grace period of Windows Vista not enough for you?
DailyTech has a quick article online which explains how you can reset (or rearm) the countdown timer. You are able to do this three times for a total of 120 days of usage before you'll need to activate your install of the new OS with your CD key.
What if 30 days isn't enough time to make a decision whether pay extra to upgrade (or legitimize) a copy of Windows Vista? After all, an operating system is the basis for anyone's computing experience, and is that is unlikely to change until the next big release rolls around - which Bill Gates confirmed could be 2010 or 2011. For those people who need more than just 30-days to evaluate their copy of Windows Vista, Microsoft has an interesting but rarely talked about feature to extend the trial license.
Unlike other methods that freeze the activation countdown timer, extending the trial to 120 days requires no hacking, cracking or illegal third party files.