Sometimes, a patch can be almost as bad as the problem it fixes.

That appears to be the case with problems that arose with Microsoft's (NASDAQ: MSFT) most recent set of bug patches, which for some users triggers a "black screen," according to U.K. security firm Prevx.

The security company posted a blog entry late last week describing users' problems and offering a free fix that it says will work for most affected users.

"There appear to be many causes of the black screen issue," the post, by Prevx spokesperson Dave Kennerley, said. The symptoms, though, stand out.

"After starting your Windows 7, Vista, XP, NT, W2K, W2K3 or W2K8 PC or server, the system appears normal. However, after logging on there is no desktop, task bar, system tray or side bar. Instead you are left with a totally black screen and a single My Computer Explorer window. Even this window might be minimized making it hard to see," the post continues.

Prevx's blog post claims that there are some 80 million results for a Google search on the topic. A quick search by InternetNews.com, however, found only about 36 million results and most of those were for earlier black screen problems, not for the latest batch -- making it difficult to gauge the scope of the issue.

A Microsoft spokesperson responded via e-mail with the company's usual boilerplate statement that it is "investigating reports that its latest release of security updates is resulting in system issues for some customers."

"Once we complete our investigation, we will provide detailed guidance on how to prevent or address these issues," the statement continued.

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In other words, Microsoft hasn't yet committed to releasing a patch for the patch that appears to be causing the problems, though it's looking into it.

The root of the issue appears to be caused by a change in handling some Windows registry entries that were added by the latest set of Patch Tuesday fixes released on Nov. 10, according to Prevx.

"The cause of this recent crop of Black Screens appears to be a change in the Windows Operating Systems lock down of registry keys," Kennerley's post said. "This change has the effect of invalidating several key registry entries if they are updated without consideration of the new ACL [access control lists] rules being applied. For reference the rule change does not appear to have been publicized adequately, if at all, with the recent Windows updates."

For users who are afflicted with the black screen problem, Prevx says it has a fix that will fix the most common cause. The firm is offering a free downloadable tool.