If you like to keep multiple Windows installations running for software testing or go-to backup purposes, you’ll be glad to know that Windows 7, like its Vista forbearer, can be run without an activation key for up to 120 days.
The command line code (slmgr -rearm) that could be entered at the end of three different 30-day periods to give Vista 120 days without activation works just the same in Windows 7, and Microsoft even confirms the work-around to PC World, calling it a “grace period” for new users.
Windows 7 Will Run 120 Days for Free [PC World]