Software giant Microsoft is pondering what the hell went wrong when a stability upgrade it released last week for Windows 7 made its flagship OS less stable.

According to a thread on Microsoft TechNet, the update (KB977074) causes screen freezes and hangs the boot screen. At shutdown the PC often hangs with a message that a program is still running but if you force the program end it does not work. In the end all you can do is swear and hit the power button.

Some people are complaining that they got a message indicating they needed to validate Windows 7. The revalidate succeeded, but was a pain in the bottom. Windows media centre shuts down at will.

Microsoft moderator Arthur Li responded in the thread and recommended that users experiencing the issue should update their BIOS and their hardware drivers, disable any antivirus program they have installed, restart their computer with a clean boot, and try installing the update again.

Microsoft is apparently still scratching its head to work out what is causing the problem. It is not convinced that it is the patch.