As most people now know, unlike previous versions of Windows, you can actually install Windows Vista without specifying a product key during installation. Once installed you're given a 30 day grace period to evaluate the operating system before you have to enter a product key and activate with Microsoft.

There is however, a way to legally extend the grace period up to 120 days using a method that has been put in place by Microsoft.

Here's how you do it:

- Login to Vista as Administrator
- Bring up the Run dialog box (do this by using Windows Key + R on your keyboard)
- Type cmd and press Enter. This brings up the command prompt.
- In the command prompt window, type slmgr -rearmand press Enter.
- Then reboot your computer.

You can do this 3 times. Each time extending your grace period by 30 days giving you a total evaluation period of 120 days.