Microsoft is allowing those curious to try out Windows 7 to do so for free for 120 days without an activation key.

Microsoft will users to keep multiple Windows installations running for software testing or go-to backup purposes.

Like its predecessor, Windows Vista, users can be run without an activation key for up to 120 days.

The command line code slmgr -rearm can 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.

You can run the -rearm trick a total of three times, said Woody Leonard, a contributing editor to Windows Secrets and the author of several computer books, including Windows Vista All-in-One Desk Reference for Dummies.

 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, he said, according to PC World.