Windows 7 Release Candidate is Available Today. Windows 7 expires on June 1st, 2010, but it does have one caveat, as of March 1st, 2010 it shuts down every 2 hours to remind you that it is going to stop working on June 1st.
Here is the quote:
Watch the calendar. The RC will expire on June 1, 2010. Starting on March 1, 2010, your PC will begin shutting down every two hours. Windows will notify you two weeks before the bi-hourly shutdowns start. To avoid interruption, you'll need to install a non-expired version of Windows before March 1, 2010. You'll also need to install the programs and data that you want to use.