Jeff Atwood over at Coding Horror tells you how to legally evaluate Vista for 120 days (instead of the standard 30) before entering 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
He has more, including reminding you to re-boot after doing this, and letting you know how to buy Vista and how much it's going to cost you.
As they say, read the whole thing.