Windows 7 has to be activated within a period of 30 days. You can use the following command up to 3 times to extend the activation period to up to 120 days. The Command has to be executed as Administrator user on the Windows shell:
Click on "Start" > "All Programs" > "Accessories" and then right click on "Command Prompt" and select "Run as Administrator"
In the command promt window enter the following command and hit return:
This slmgr command works also on the Test versions of Windows 7 that Microsoft offers on their website. It has to be executed before the activation period ended.