That is perfectly normal. The RC version is still Beta software, but is the final step prior to RTM. In many cases, the RC version is the same as the final RTM version. This enables any remaining bugs to be dealt with prior to the true RTM release. Both the Beta versions and RC versions will display that warning. It is to remind you that Beta software (and that includes RC) is time-limited. The RTM version will not display this warning. You will, of course, need to uninstall the Beta/RC version of SP1 prior to installing the RTM version. I suggest that it would also be a good time to do a full reinstallation of the OS as well.