You Can Install Windows Vista SP1 RC (Release Candidate) through Windows Update

From today, you can install Vista SP1 release candidate
which is one step better than Beta version (SP1). As you can get it
through Windows Update if you have already installed Vista SP1 by using
the standalone package, you have to uninstall it first. This SP1 RC is
compatible with all the systems running on one of the 36 languages
supported by Vista RTM.