Microsoft posted a tool to its download site Friday that will block automatic installations of several upcoming service packs, including Vista Service Pack 1 and Windows XP SP3.

The download includes three versions of the tool -- an executable, a script and a group policy template -- that prevents the service packs from reaching PCs via Windows Update, Microsoft's default update service.

The tool blocks Windows Vista SP1, Windows XP SP3 and Windows Server 2003 SP2 for varying lengths of time. Vista SP1 and XP SP3 can be blocked for as long as 12 months after the service packs are released in final form, while the Server 2003 SP2 blocker bars the download only through March 2008.

Vista SP1, which enters public testing Tuesday, is scheduled to wrap up in the first quarter of 2008. Windows XP SP3, on the other hand, has a delivery date of sometime in the first half of next year.

Microsoft has offered similar blockers before, most recently one last year that kept Internet Explorer 7 from automatically installing on Windows XP systems. The blocking tools are designed for businesses, particularly large corporations and organizations that require extensive testing before they deploy a service pack or operating system upgrade.

The tool lets all other updates, including monthly security patches, pass through Windows Update without modification.

Microsoft also plans to unveil Office 2007 SP1 this week, but has said nothing about a blocker for the suite update.

The Windows Service Pack Blocker Tool Kit can be downloaded from the Microsoft Web site.