As the amazing Jake told you, Windows 7 Release Candidate 1 is available to the public. Lots of people were surprised to learn that RC1 is legit until June 1, 2010. It's important to note that a few months before it expires, this install of Windows 7 will become annoying as hell. Microsoft noted:
Certainly some people be in a huff about this, but what did you expect? You're getting a fairly stable of Windows 7 for free and it doesn't expire until June 2010. Microsoft didn't release RC1 to the public out of altruism; the company wants people to upgrade to Windows 7 or buy PCs loaded with Windows 7. Personally, I think it's a pretty funny control -- auto-shutdown every two hours? That's hilarious! I'm also sure that some enterprising hacker will find a way around it.
Source via Gizmodo