Be patient, especially if you're going after one of the full install packages (which are over 500MB each).
Tried the Beta? Read This First
If you installed a beta of Windows Vista SP1 RC, uninstall it first and wait at least one hour before continuing (you might get cramps in your CPU otherwise). Read Microsoft Knowledge Base article KB936330 for details.
Before SP1, You Also Need...
When you install Windows Vista SP1 using the full install download, the system checks to see if updates discussed in KB937287 (updates servicing stack to permit the service pack to be successfully installed), KB938371 (updates installer and permits service pack to be uninstalled before the final SP1 is installed), and KB937287 (updates BitLocker support on Windows Vista Ultimate and Enterprise editions) have been installed. They've been released separately via Windows Update, but if you don't have them yet, they must be installed first. (update 12-12-07 - Don't worry, the SP1 RC installer program will install them as needed).
If you use Windows Update to get Vista SP1 RC, you must install these updates before you will see Windows Vista SP1 offered as an update and they are displayed only after you install the update for Windows Update referenced earlier.