Just read this on Softpedia:-
"The development milestones of Windows 7, be them pre-Beta or fully-fledged Beta, are time-bombed, as is the case with all Microsoft pre-release software. When it comes down to the Milestone 3 releases of the next iteration of the Windows client, the expiration date is officially set to August 1, 2009. This despite the operating system indicating via Winver that it will expire as early as July 1, 2009. With or without a product key, pre-release versions of Windows 7 including Build 6801 and 6956, but not earlier than 6801, can be used up until August 1, 2009.
The expiration date of Windows 7 is delivered under the Time Sensitive Software section of the End User License Agreement. “This software will stop running on August 1, 2009. You may not receive any other notice. You may not be able to access data used with the software when it stops running,” Microsoft informs.
Indeed, even development milestones of Windows 7 that are activated using a genuine or a leaked product key, or by any other workaround, will stop running come August 1, 2009. Still, when it comes down to activation, users needn't resort to cracks or leaked product keys. Windows 7, even in the pre-release builds, supports rearming, just like Windows Vista. This is done via the Software License Manager or SLMGR.vbs. Slmgr is a tool that, as its name implies, permits end-users to deal with the various aspects of licensing.
At the same time, on Windows Vista, Windows Server 2008, Windows 7, and Windows 7 Server (Windows Server 2008 R2) slmgr permits the prolonging of the trial period. On Windows 7, the trial can be extended to as much as 120 days via slmgr -rearm. Each execution of the command in a command prompt window will add 30 days of trial mode, but it will also reset the expiration counter to zero. In this context, users will have to execute the command as Windows 7 copies are just about to expire.
In addition to the expiration of the Windows 7 development milestone bits, August 1, 2009, is also synonymous with the expiration of the operating system's license. “The term of this agreement will expire on August 1, 2009. You have the right to use the software under the license terms of this agreement during this period. If you do not have a product key, you may need to reinstall the software every 30 days.” Making Windows 7 Build 6956 Survive Until Expiration - Product key or no product key - Softpedia