Microsoft is said to be investigating reports that a recently-released security update is causing computers to lock up and display a "mostly" black screen with no desktop, taskbar, sidebar or anything else - just a My Computer explorer window.

The issue is said to affect Windows 7, Vista and XP and relates to a set of six updates that were released on 10 November for Windows, Office and other products. Given that Microsoft is trying to inspire consumer confidence in Windows 7 after the public opinion disaster that was Vista, this won't be making Microsoft executives very happy.

PC World has a quote from CEO of computer security types Prevx who says that the issue is "massively debilitating" for those who are experiencing it. Prevx reported the problem on 27 November and offers a free tool that may fix the issue.

Microsoft would only say: Microsoft 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".

UPDATE: Microsoft has got in touch, saying: "Microsoft can confirm that it was not the November security update that has caused this very small scale reported screen freeze issue, but is in fact more likely to be malware". The company has written a blog post on the subject, and Prevx has apologised.