Task Scheduler 1.0 is included with Windows 2000, Windows XP and Windows Server 2003. It runs as a Windows Service, and the task definitions and schedules are stored in binary.job files. Tasks are manipulated directly by manipulating the .job files. Each task corresponds to single action. On Windows 95, Windows 98 and Windows Me, the Task Scheduler runs as an ordinary program, mstask.exe. It also displays a status icon in the notification area on Windows 95 and Windows 98 and runs as a hidden service on Windows Me, but can be made to show a tray icon. On Windows 2000, Windows XP and Windows Server 2003, it is implemented as a Windows service. Computer programs and scripts can access the service through six COMinterfaces. Microsoft provides a scheduling agent DLL, a sample VBScript and a configuration file to automate Task Scheduler.
In addition to the graphical user interface for Task Scheduler in Control Panel, Windows provides two command-line tools for managing scheduled task: at.exe (deprecated) and schtasks.exe. However, at.exe cannot access tasks created or modified by Control Panel or schtasks.exe. Also, tasks created with at.exe are not interactive by default; interactivity needs to be explicitly requested.
Task Scheduler 2.0 was introduced with Windows Vista and included in Windows Server 2008 as well. The redesigned Task Scheduler user interface is now based on Management Console. In addition to running tasks on scheduled times or specified intervals, Task Scheduler 2.0 also supports calendar and event-based triggers, such as starting a task when a particular event is logged to the event log, or when a combination of events has occurred. Also, several tasks that are triggered by the same event can be configured to run either simultaneously or in a pre-determined chained sequence of a series of actions, instead of having to create multiple scheduled tasks. Tasks can also be configured to run based on system status such as being idle for a pre-configured amount of time, on startup, logoff, or only during or for a specified time. XPath expressions can be used to filter events from the Windows Event Log. Tasks can also be delayed for a specified time after the triggering event has occurred, or repeat until some other event occurs. Actions that need to be done if a task fails can also be configured. The actions that can be taken in response to triggers, both event-based as well as time-based, not only include launching applications but also take a number of custom actions. Task Scheduler includes a number of actions built-in, spanning a number of applications; including send an e-mail, show a message box, or fire a COM handler when it is triggered. Custom actions can also be specified using the Task Scheduler API. Task Scheduler keeps a history log of all execution details of all the tasks. . Windows Vista uses Task Scheduler 2.0 to run various system-level tasks; consequently, the Task Scheduler service can no longer be disabled (except with a simple registry tweak).
Task Scheduler 2.0 exposes an API to allow computer programs and scripts create tasks. It consists of 42 COMinterfaces. The Windows API does not, however, include a managed wrapper for Task Scheduler though an open source implementation exists. The job files for Task Scheduler 2.0 are XML-based, and are human-readable, conforming to the Task Scheduler Schema. Although possible, Microsoft advises not to create the job files by hand, and instead, use the Task Scheduler API.
New security features, including using Credential Manager to passwords for tasks on workgroup computers and using Active Directory for task credentials on domain-joined computers so that they cannot be retrieved easily. Also, scheduled tasks are executed in their own session, instead of the same session as system services or the current user.
Ability to wake up a machine remotely or using BIOS timer from sleep or hibernation to execute a scheduled task or run a previously scheduled task after a machine gets turned on.
Ability to attach tasks to events directly from the Event Viewer.
The Task Scheduler service works by managing Tasks; Task refers to the action (or actions) taken in response to trigger(s). A task is defined by associating a set of actions, which can include launching an application or taking some custom-defined action, to a set of triggers, which can either be time-based or event-based. In addition, a task also can contain metadata that defines how the actions will be executed, such as the security context the task will run in. Tasks are serialized to .job files and are stored in the special folder titled Task Folder, organized in subdirectories. Programmatically, the task folder is accessed using the ITaskFolder interface or the TaskFolder scripting object and individual tasks using the IRegisteredTask interface or RegisteredTask object.
On Windows 2000 and Windows XP, tasks assigned to run with SYSTEM privileges do not function when the computer is prepared for disk imaging with sysprep. Sysprep changes the security identifier (SID) to avoid duplication but does not update scheduled tasks to use the new SID. Consequently, all SYSTEM scheduled tasks fail to run on the imaged computers. There is no solution for this problem but one may reschedule the tasks to work around the issue.
On Windows Vista or Windows Server 2008, where Service Pack 2 is not installed, the next execution time displayed in Task Scheduler may be wrong.
On Windows Vista, 7, 2008, and 2008 R2: The MMC Component says that you are running "Task Scheduler 1.0" when in fact you are running 2.0, this is a trivial bug so it wasn't noticed, and is likely due to the re-write of the task scheduler. The version has been corrected to 2.0 in Windows 8 and in 2012.