I thought this might help people who are having problems updating Vista.
Here`s a discription of what it does :
Hope this helps someone
This article describes the System Update Readiness Tool for Windows Vista-based computers and for Windows Server 2008-based computers. Microsoft released the System Update Readiness Tool to resolve certain conditions that could cause installing updates and other software not to work. You do not have to manually run this tool. This tool is offered automatically through Windows Update to computers that have a condition that the tool could resolve.
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After you download the System Update Readiness Tool, it runs a onetime scan for inconsistencies that might prevent future servicing operations. This scan typically takes less than 15 minutes to run. However, the tool might take significantly longer on some computers. The Windows Update progress bar is not updated during the scan, and progress seems to stop at 60% complete for some time. This behavior is expected. The scan is still running and you should not cancel the update.