is the C1Halt function called from the Idle process? If yes, this nothing you should worry about. This is a powersaving feature of CPUs (http://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface). ok, because I can't see which driver is causing this you must use the old "try and look" way be running a clean boot: http://support.microsoft.com/kb/929135 Does a Clean boot help? If yes, a driver from your application is causing it.