Microsoft has confirmed that it is investigating a problem described as the "black screen of death", which affects its latest operating system.
The error means that users of Windows 7 see a totally black screen after logging on to the system.
The firm said it was looking into reports that suggest its latest security update, issued on 10 November, was the cause the problem.
Reports suggest the flaw also affects Vista, XP and other systems.
Software firm Prevx, which has issued a fix for the problem, says "millions" of people may be affected.
"Users have resorted to reloading Windows as a last ditch effort to fix the problem," the firm's David Kennerley wrote in a blog post.
"We hope we can help a good many of you avoid the need to reload."
