A security update from the AVG security software has crippled PCs running Windows XP.

The update mistakenly took a critical system file as a threat and deleted it.

As a result, users were either unable to boot-up their PCs or saw their laptops continually boot-up.

The update issued last weekend mistakenly fingered the "user32.dll" file as a Trojan horse.

As a result, the updated AVG software put the legitimate file in quarantine, leading to a computer that would not work.

AVG said on its support website, "In case you are not able to run your Windows XP operating system after AVG 8.0 virus definition update, it may be caused by a false positive on a specific 'user32.dll' system file.

"The file was moved to the AVG Virus Vault and deleted. Therefore it is not possible to start Windows."

To get round the problem, affected users have to use Windows XP's Recovery Console, disable several AVG services and restore the user32.dll file by copying it from the operating system's install CD. This is not an easy task for many users, particularly if they cannot locate their recovery disk.