If you want continue playing with Windows 7 PDC installation after 30-days trial period ended, you can “rearm” it and use for a bit longer period of time using this simple approach:

  1. Install Windows 7 without any product activation key.
  2. After installation is completed, use the Windows 7 for 30 days and wait for the remaining days left to activate Windows counting down to 0, or almost zero.
  3. When the activation grace period (or evaluation trial period) is almost expired or ended, log on to Windows 7 desktop, and open a Command Prompt window (i.e. type Cmd in Start Search and hit Enter).
  4. Type any of the following commands into the command prompt, and then hit Enter:sysprep /generalize

    slmgr.vbs –rearm

    rundll32 slc.dll,SLReArmWindows

    slmgr /rearm

  5. Reboot Windows 7 to enjoy another 30 days of free usage without worrying about activation nor even need to crack Windows 7.
  6. When the activation grace period countdown timer almost running down to 0 again, repeat the ‘rearm’ trick to enjoy another 30 days of Windows 7 for free. User can run the rearm command for maximum of 3 times.

After this you will require to activate it with proper product key or install other trial version, that probably will be released soon.

from http://www.mydigitallife.info/2008/11/06/how-to-rearm-and-extend-free-usage-activation-grace-period-of-windows-7-to-120-days/