Vista SP1 will be distributed via Microsoft Update

Source: System Center Essentials Team Blog

Windows Vista SP1 will be released as an update on Microsoft Update (MU). The patch is very large and there is a bug in Windows Server 2003 in the WinVerifyTrust API that will cause signing validation to fail.

What this means is that once you approve this update on a System Center Essentials 2007 server on a Windows Server 2003 server, every time the server sync’s from MU it will redownload the package, fail the cert validation, and so the download will fail. The problem will continue until you install the WinVerifyTrust patch on the System Center Essentials server. This patch is a hotfix (not a public GDR), so is not intended to be widely distributed. We recommend it only be installed on the System Center Essentials server itself.

You can obtain this hotfix here:

Windows Server Update Services cannot download large Windows update files in Windows Server 2003
http://support.microsoft.com/kb/888303/en-us