How to extend Win7's trial to a full four months

In a nutshell: If you install Windows 7 and don't enter an installation key, the 30-day activation clock starts.
To see how many days you have 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:

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• 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 -rearmNote the space after slmgr and the hyphen in front of rearm.
• Step 3: Restart Windows 7.

Once the OS restarts, the Properties dialog described above will 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.

If you perform a -rearm at the end of each 30-day period, you end up with 120 days of full, unfettered Windows 7 use without having to supply an activation key in the interim.