Windows Vista is the first windows operating system to let you use a trial version for 30 days before buying. It's a really great move for Microsoft, especially in light of how little benefit you get from upgrading to Vista in the first place. There's a soon to be well known trick that can extend the trial period up to 120 days.
If you right-click on the computer icon on the start menu, you'll see the basic system information window, and you'll also see the amount of days you have left to activate:
Looks like I only have 12 days left… or do I?
If you are not locked out of your computer
Open up an administrative mode command prompt by typing in cmd into the start menu search box, and then hit the shortcut Ctrl+Shift+Enter to launch in administrative mode.
Now run this command from the prompt:
It will take a little while, but eventually a dialog will pop up telling you to reboot. Do so, and you'll see that the activation period has been reset to 30 days.
Note that if you want to get a full 120 days out of this tip that you need to use this command on day 29 or 30 of your trial mode. You can only use this command 3 times before Microsoft smacks you down.
If you are already locked out (Reduced Functionality Mode)
You can still reactivate Vista if you have more resets left. If you've already used up all 3, then you are out of luck, but if not, you can follow these steps to unlock your computer again.
First, Open up Internet Explorer, and type C:\ into the address bar. You'll be prompted for UAC.
Now type in C:\Windows\System32\ into the address bar, and find the cmd.exe executable. Right-click it and choose "Run as Administrator".
Now use the same method as above (slmgr -rearm).
You should see the message saying that the computer is rearmed. You'll probably want to restart the computer at this point.
Getting a Trial Copy of Windows Vista
Windows Vista is the first Microsoft operating system to let you have a trial period. One of the questions that I get asked constantly is "Where do I get a copy of Vista?". This answer is less obvious.
If you have an MSDN license, you can download it from there and run it in trial mode. Most people don't have MSDN, however.
If you know somebody that has a copy of Vista, you can borrow the CD and use it to install in trial mode. As long as you aren't trying to use their serial number, there shouldn't be a problem with this.
There are various "other" ways to get a copy of the Vista CD. If you don't know what I mean, you should ask the person that everybody knows has a huge collection of downloaded content. I suspect they can hook you up, but remember it's just a trial copy. I don't advocate piracy.
Alternatively, you could buy a copy.
Addressing the 1 Year Activation Patch
There's been a debate about a 1 year activation hack that can allow you to run Windows Vista for up to 1 year without activating. As far as I can tell, this is completely bogus, and shouldn't even be attempted. Honestly, it's best to just buy a new computer with Vista already installed.
Addressing the "TimerStop" hack
There is a hack that will allow you to run Windows Vista forever without registering. This hack is obviously illegal, unsupported by Microsoft, and likely to break with future patches to Vista. I have verified that this works with some friends. Although I have not personally used it, I do not believe it should be used, and I would advise strongly against doing so.