If you managed to get a copy of Windows 7 disc from someone who purchased it or you manage to download from torrents, newsgroups; Microsoft lets you try it for 30 days without activating it online. Well now, Windows 7 users will be able to use the software free for 120 days before they have to officially buy it.
The grace period can be extended up to three additional times — to a total of 120 days — using a Microsoft-supplied utility called the Software License Manager (slmgr).
Simply go to – Start menu, selecting All Programs, then Accessories, then Command Prompt. Once the Command Prompt appears, type "slmgr -rearm", without the quote marks. Once the computer is rebooted, the activation calendar will have been reset to zero, giving users another 30 days of free use.