On May 31, 2007, pre-release versions of Windows Vista will expire. If you are running a pre-release version of Windows Vista (Beta 2, RC1, or RC2) you will begin to receive notifications about the upcoming expiration on May 18, 2007. To avoid work disruption and the loss of data, it is strongly recommended that PC users running any of these pre-release versions of Windows Vista migrate their PCs to the final version of Windows Vista prior to May 31.

More information at http://www.microsoft.com/windows/products/windowsvista/preview.mspx on your possible upgrade paths.