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 could probably get around that, but it would still be a bore. Also, since there will only be support for the final product, I think it's well worth the £50 or £100 for something you will use every single day.