Source:

Microsoft has made available the Windows 7 RC and is now available for public download from Microsoft servers. The Release candidate will be available till July 2009 and there is no limit to the product keys, so you have enough time to download the RC.

Download Link :
http://www.microsoft.com/windows/windows-7/download.aspx
 
Minimum system requirements : http://www.nirmaltv.com/2009/05/02/windows-7-system-requirements/
 
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