Microsoft confirmed that it is investigating a problem described as the "black screen of death", which affects Windows 7, XP.

According to the reports, after a user is starting a Windows 7, Vista, XP, NT, W2K, W2K3 or W2K8 PC or server system, it appears normal. However, after logging on there is no desktop, task bar, system tray or side bar. Instead users 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.

Software firm Prevx, which has issued a fix for the problem, says "millions" of people may be affected.

The firm said it was looking into reports that suggest its latest security update, released on Tuesday 25 November, cause the problem.

"If you have these symptoms you can safely try our free Black Screen Fix. It will fix the most common cause we have seen of this issue," David Kennerley (Prevx support) wrote at its blog. He also outlines the installation procedure for the fix.

"I must stress that this tool will not fix all black screen issues. There can be many causes. But if your black screen woes began in the last 2 weeks after a Windows update or after running any security program (including Prevx) to remove malware during this time then this fix will have a high probability of working," Kennerley added.

Mr. Kennerley also claims that the cause of this recent crop of Black Screen appeared to be a change in the Windows Operating Systems lock down of registry keys. "This change has the effect of invalidating several key registry entries if they are updated without consideration of the new ACL rules being applied. For reference the rule change does not appear to have been publicised adequately, if at all, with the recent Windows updates," he added.

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.

Microsoft said that people who are affected by the problem should contact its customer service line.