Microsoft has broken its normal monthly patch cycle in order to issue a critical security update for Windows PCs.

The Redmond software giant rarely breaks out of its scheduled updates – but when it has in the past it has been to plug major problems when they come to light.

The patch apparently addressed an exploit affecting the 'remote procedure call' (RPC) which allows an .exe file to be run from a remote connection.

Critical

The problem was noted as 'critical' on Windows 2000, Windows XP and Windows Server 2003, denoting that the exploit needs little or no help from the user to infect machines, and was classed as 'important' on Windows Vista and Windows Server 2008.

Updates can be grabbed from http://update.microsoft.com/or by enabling automatic updates.