Vista Service Pack 1 Release Candidate Refresh.

Microsoft released this "Release Candidate refresh" only one month after the initial RC made its debut. It claimed to want additional information from testers, which indicates that even more bugs need to be fixed. But what does it really do, and is it really that large? Most importantly, do you need it?

Vista SP1 RC Refresh requires several updates before the actual Release Candidate can install.
(Credit: CNET Networks, Inc.)

Before we get to the nitty-gritty of whether or not it's worth downloading, the download itself needs a little explanation. First off, Microsoft warns that the SP1 RC and the Refresh should not be installed on primary or mission critical machines. Make of that what you will. Check out the changelog to Vista SP1 RC. If it's something you think your Vista installation needs, carry on, but keep in mind that as a Release Candidate it's not intended to be a fully stable version.
So, if you installed the original SP1 RC from December, you'll need to uninstall it. Go to your Control Panel/Windows Update/View Installed Updates, and then follow the instructions for uninstalling. The official Microsoft instructions warn users to wait an hour after uninstalling Vista SP1 RC installing the Refresh because, "the installer service needs to clean up and complete the uninstall prior to installing the RC. Failing to do this can result in installation errors when installing the RC version." Also note that you will be required to uninstall the SP1 RC Refresh before installing the full SP1 when it comes out. That's a lot of installing followed by uninstalling followed by installing to do.

Source: CNET.COM