Microsoft is effectively giving away Windows 7 free for a year with the launch of the Release Candidate. The Release Candidate is now available to MSDN and TechNet subscribers, and will go on unlimited, general release on 5 May. The software will not expire until 1 June 2010, giving testers more than a year's free access to Windows 7.
The Release Candidate includes all the features to be available in the final version. Even so, keep in mind that this is beta software. Do not entrust your critical data and applications to beta software. I’d suggest staying with an older operating system for some time until Microsoft can get most of the bugs out. However, if you have an extra computer, either a physical system or a virtual one, the free version of Windows 7 is a great way to experiment with the new operating system and learn about its strengths and weaknesses.
Microsoft recommends NOT upgrading from earlier versions of Windows. Instead, the company recommends reformatting the hard drive and starting with a fresh installation.
You can read more at http://www.pcpro.co.uk/news/252181/windows-7-free-for-a-year.html.