If you are about to install Vista SP1 RC consider this.
"Windows Vista (TM) Evaluation Copy. Build 6001" will show on the bottom right of your desktop.
Notice the following from the included readme document.
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.
There is a hack to remove the watermark but it's definitely unsupported by Microsoft.