Microsoft allows extending Windows 7's standard 30-day product activation grace period to a full 120 days, using a little-known command in a software licensing utility that comes with the operating system. Windows 7 RTM’s currently availabe for download to subscribers of MSDN, TechNet services for developers and tech professionals, and beta testers.
Here’s the trick: If you install Windows 7 and don't enter an installation key, the 30-day activation clock starts. To see how many days you’ve left, click Start, right-click Computer, and choose Properties. At the bottom of the dialog under Windows Activation, you'll see the number of days left in your trial period. When that number gets perilously close to zero, you can extend the free period another 30 days via the following steps:
Step 1: Click Start, All Programs, Accessories. Right-click Command Prompt and choose Run As Administrator. Enter your administrator password.
Step 2: Type the following command and press Enter: slmgr -rearm
Step 3: Restart Windows 7.
(Once restarted, the Properties dialog described above’ll indicate that Windows 7's activation grace period has been reset to a full 30 days. You can run the -rearm trick a total of three times.