read this from M$ Windows Vista SP1 RC is time-limited software. Windows Vista SP1 RC will no longer operate after June 30, 2008 and should be uninstalled prior to that date. After installation your desktop will show the text “Evaluation Copy”. This does not mean that your system is no longer genuine but that you have installed a test version of software which is time-limited. The “Evaluation Copy” text will appear on your desktop after the second reboot after installing the service pack. Likewise, the “Evaluation Copy” text will remain on your desktop during the first reboot after uninstall the service pack, but will disappear after the next reboot. If you install Windows Vista SP1 RC it is strongly recommended that you install the final version of Windows Vista SP1 when it is released. To do this, you will be required to uninstall this RC version of Windows Vista SP1.
That would happen if you don't use the tokens.dat trick, if you do you will not get the watermak and there wont be any expiration day....