Microsoft warns that the SP1 RC and the Refresh should not be installed on primary or mission critical machines, and rightfully so. This is not a basic Service Pack, but a beta, and should only be installed if you feel comfortable making serious and potentially severe adjustments to your machine.
If you installed the SP1 Release Candidate from December 2007, uninstall it before installing this RC Refresh. To install the Refresh, download the self-extracting EXE and right-click on the CMD file. Choose Run as administrator. This creates a Registry key to allow the update to install. Now go back to the Control Panel and go to System and Maintenance/Windows Update, turn automatic updating on, and select Check for updates. This way you stand a better chance of Vista detecting the SP1 RC Refresh on Microsoft's servers. Microsoft's documentation on the installation warns that it can take up to an hour to detect the patch.
Vista will automatically download and install two patches before running the SP1 Refresh, and Enterprise and Ultimate versions will install a third patch to make their BitLocker Encryption functions compatible. Users have noted that the biggest difference seems to be on laptops running Vista, where battery life increased. However, as this is a beta and a buggy and hard-to-install one at that, we recommend waiting until a more stable version is released.