Anecdotal evidence suggests that the problem is triggered by a conflict between the latest Windows security updates and previous security updates from Microsoft, causing the PC to either go into a constantly rebooting sequence or simply freeze up.

Some machines are reported to lock up with a black screen, although similar machines with the the same batch of operating system software either seem to freeze up or constantly reboot, Infosecurity notes.

IT security vendor Prevx claims to have spotted the problem and has issued a patch for the issue, although Microsoft has not commented on the actual cause of the security update problem.

Prevx said it picked up on the Windows security update issue after users of its security software reported problems late last week.

Unconfirmed reports suggest that the Access Control List (ACL) - the user permission system at the heart of Windows - may be experiencing problems following a series of Microsoft security updates.

In a blog posting, David Kennerley of Prevx said that there are many causes of the black screen issue.

"The symptoms are very distinctive and troublesome. After starting your Windows 7, Vista, XP, NT, W2K, W2K3 or W2K8 PC or server the system appears normal. However, after logging on there is no desktop, task bar, system tray or side bar. Instead you are left with a totally black screen and a single My Computer Explorer window. Even this window might be minimised making it hard to see", he said.

"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."