Microsoft is urgently investigating a problem affecting its much-vaunted new operating system, Windows 7.
The error, described as a "black screen of death", results in the user being confronted with a dark screen after logging on to their PC. The software company said it was looking into reports that a security update issued last month had created the problem.
According to reports, the flaw also affects Vista and XP as well as other older Windows systems. The software firm Prevx said "millions" of people could be affected, and that many had resorted to re-installing Windows 7 to resolve the problem.
Microsoft said people who are affected by the bug should contact its customer service line. So far it has not issued a fix for the problem, which causes the desktop, task bar, system tray and side bar to disappear.
The nickname "black screen of death" is an update of the similar "blue screen of death", which often appears when Microsoft operating systems crash.