Extend Windows 7 & Windows 2008 Grace Period.

On Installation Windows 7 requires licence activation. By default the grace period to do this is 30 days.

If for legitimate reasons this needs to be extended, it is possible to reset this a maximum  of 3 times.  Resulting in a total 120 days grace period.

This option is activated by running a command prompt session in administrator mode and entering the following;

slmgr.vbs -rearm

A message box will be displayed and the machine will need to be restarted.

This is a Microsoft supported option, and does not violate the EULA
This also applies to windows vista.