One of the ways Vista’s security was improved was by separating system services and user applications into separate ’sessions’. Keeping the system services isolated helps to better secure them, but also makes any interactive interface unavailable to the user. That’s where the Interactive Services Detection service comes in. When a service needs to interact with the user, Interactive Services Detection presents a dialog that will switch the user to the session where the service is running so they can interact with the service. For an excellent, detailed description of this, see this post. Many sites recommend disabling this service, but doing so will result in you not being able to interact with any services that require your attention. This service is run manually by default, so there is little point to disabling it unless you don’t want to be bothered by important information from the software you may be trying to run.