Microsoft says that patches for a Windows Update lockup problem do fix the flaw, even though users still see their computers' CPUs maxed out at 100 percent.

The "svchost/msi issue" issue has plagued Windows users, particularly those running Windows XP, for months. When users tried to retrieve security patches from Windows Update or Microsoft Update, manually or via Automatic Updates, their PCs' processors would hit 100 percent and stay there, making the machines unusable. A two-part patch available for manual download was posted earlier this month, but users who installed it said the repair was worthless: They still saw CPUs redlining.

On Tuesday Microsoft began offering the first half of the update as a high-priority, non-security fix to Windows users through its various patch services. "This update is the first part of a two-part fix that is the comprehensive solution to the problem," Microsoft stated in an advisory published on its Web site. "In June, another update will involve the Windows Update client. The update for the Windows Update client will also be automatically offered through Automatic Updates."

Although users will have the fix in place only after both parts are installed - making anyone who relies on Automatic Updates vulnerable to the svchost/msi bug until sometime next month - even users who downloaded both pieces manually were initially unconvinced that it worked. "Doesn't appear that the fixes address the issue I experience on multiple machines," said one user in a posting to the Windows Update support forum two weeks ago.

This week, Microsoft said that the full fix really does work, appearances notwithstanding.

"It's important to note that with the MSI fix and the new client installed, the CPU may still go near 100 percent, but the system should still be responsive and not lock up," said Bobbie Harder, a program manager on the Windows Server Update Services (WSUS) team, on the group's blog.

"If another task requires CPU cycles they will be shared, but if the system is idle, MSI will use the full cycles available. If a task is running at the same time as MSI, the system may be slightly slower, but should still be responsive during this time."

The patches, Harder added, are meant to address locked up systems, not knock down CPU usage. "CPU spikes during some scans are expected, machine unresponsiveness is not," said Harder. "If your [sic] watching the process monitor, you will still see 100 percent CPU during some scans and this is expected behaviour."

That high, and expected, CPU load, was what prompted users to erroneously report the updates didn't do the job, Harder intimated in a post the next day. "After installing the new 3.0 client and the MSI fix, we tested scan times, system responsiveness and CPU utilization on a variety of systems running multiple applications simultaneously," Harder said.

"There was the expected CPU usage spike in the task manager, however all the systems remained responsive. We are seeing many more posts in blogs, forums and newsgroups where users are reporting these installs alleviate these issues, while the CPU consumption does temporarily spike as expected."

A Microsoft support document contains links to the appropriate downloads for Windows 2000, Windows XP, and Windows Server 2003. Windows Vista is not affected by the svchost/msi issue.