You can install and use Windows 7 legally without providing the product key for 30 days. During this 30 day evaluation period (grace period) activation is not required and you can use Windows 7 in full functionality.

After the evaluation period, however, activation is required in order to continue using Windows 7. 

The good news is that Microsoft engineers equipped Windows 7 with a secret "rearm" key that you can use to extend Windows 7 activation for another 30 days. You can use this key for up to 4 times which makes 120 days of evaluation period, that doesn't require activation.

To extend your Windows 7 activation for another 30 days legally do the following when your 30 activation period is going to expire:

1. Open Windows Command Prompt (type cmd and hit Enter in the Start Search box)
2. Type the following command and hit enter:

   slmgr /rearm
   

3. Reboot Windows 7 and you're done.

Your Windows 7 activation has now been extended for another 30 days. Enjoy!