RunOnce keys are used to launch a service or background process whenever a user logs into Windows. Once the application is launched for the first time its entry will be removed from the Registry so it does not run again when the user logins again. Applications here are designed to be used primarily by Setup programs. (If there a exclamation point preceding the value of the key, the application will not be removed until after it completes executing, otherwise it will be removed before the program runs.)