Microsoft says that a problem with its Windows operating system, dubbed the 'Black Screen of Death' is not due to its latest security update.
Some Windows users are confronted by a totally black screen after they log on to their system. It was thought that a security update on 10 November had been the cause, although this has now been discounted.
Reports suggest the problem also affects Windows Vista and XP, which experts are putting down to malware.
In a blog posting on its security site, Microsoft said that it had investigated the claims and "found that our November Security Updates are not making changes to the system that these reports say are responsible for these issues".
The firm said that the behaviour was associated with malware, such as Daonol, and that this was probably the root cause.
Software firm Prevx, who had said the November update may have been to blame, have since retracted their statement, saying it had been a "challenging issue" to identify the cause.
"Having narrowed down a specific trigger for this condition we've done quite a bit of testing and re-testing on the recent Windows patches including KB976098 and KB915597 as referred to in our previous blog," the firm wrote on its website.
"Since more specifically narrowing down the cause we have been able to exonerate these patches from being a contributory factor."
Prevx apologised to Microsoft for "for any inconvenience" its earlier claims may have caused.
The firm has issued a fix for the problem, which is says could affect "millions" of computers.
"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."
Unknown problem
The firm said its fix did not work in all cases.
"There can be many causes," said Mr Kennerley.
"But if your black screen woes began in the last two weeks... or after running any security program (including Prevx) to remove malware during this time, then this fix will have a high probability of working."
Mr Kennerly said the firm had identified "at least 10 different scenarios which will trigger the same black screen conditions".
"These appear to have been around for years now," he said.
The firm reports that the problem affects editions of Windows 7, Vista, XP, NT, and Windows 2000.
Microsoft said that people who are affected by the problem should contact its customer service line.
The "black screen of death" moniker is a play on the "blue screen of death", which appears when Microsoft operating systems crash.