This tool is being released because an inconsistency was found in the Windows servicing store which may prevent the successful installation of future updates, service packs, and software.

Once you download the tool, it scans for inconsistencies that may prevent future updates, service packs and servicing operations. This scan may take 15 minutes or longer on some computers and also progress seems to stop at 60%, So don’t cancel the update, the scan is still running.

You can get the tool from Windows Update or you can download from the Windows Update web site.

Windows Vista and Windows Server 2008

• Download the System Update Readiness Tool (KB947821) [September 2009] package now.
• Download the System Update Readiness Tool for x64-based Systems (KB947821)[September 2009] package now.

Windows 7 and Windows Server 2008 R2

• Download the System Update Readiness Tool for Windows 7 (KB947821) package now.
• Download the System Update Readiness Tool for Windows 7 for x64-based Systems (KB947821) package now.
• Download the System Update Readiness Tool for Windows Server 2008 R2 for x64-based Systems (KB947821) package now.

The tool is available to customers running genuine Microsoft Windows only.

Source: Microsoft