Today, Microsoft made available the public release of Windows Vista Service Pack 1 Release Candidate 1.
Service Pack 1 includes several improvements to the operating system, including stability and performance fixes as well as many new features and enhancements. As with most service packs, this release also includes all updates previously released for the operating system.
The service pack can be downloaded via Microsoft Download Center or via Windows Update after applying a system patch (see links below).
As this is a pre-release, there is an evaluation period. If you install the service pack, it will expire after June 30th, 2008. However, the final version of Service Pack 1 is expected to be released in January.
Note: If you are running a 64-Bit edition of Windows Vista, the service pack can only be downloaded from Windows Update.