Windows 7 isn’t as free as you thought.
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.
via Windows 7 Release Candidate Customer Preview Program.