If you'd like to Beta test Windows Vista Build # 5536 (Pre RC1) you can for a limited time only download it from http://download.windowsvista.com/preview/prerc1/en/download.html
the download link will go away when 1,00,000 downloads have been made - if you want it, you'd better make it quick! If you've missed it - dont worry - RC1 is around the corner.