I found this in a few different places around the 'net - hopefully it's helpful:
Prevent a User From Running or Stopping a Scheduled Process in Windows XP
This step-by-step article describes how to prevent a user from running or stopping a scheduled process in Windows XP.
There are occasions where is not advisable to run a scheduled process before the intended date and time, or when an administrator doesn't want a user to add new scheduled processes to the computer. You can use the following procedures to enable maximum control over what can be done with the Scheduler service.
- Click Start > Run, type mmc, and then click OK.
- On the File menu, click Add/Remove Snap-in. On the Standalone tab, click Add.
- In the Available Standalone Snap-ins list, click Group Policy, and then click Add.
In the Select Group Policy object box, click Local Computer to edit the local Group Policy object, or click Browse to find the Group Policy object that you want. - Click Finish, click Close, and then click OK. The Group Policy snap-in opens the Group Policy object for editing.
- Expand either the User Configuration or Computer Configuration branch, and then expand Administrative Templates, Windows Components, Task Scheduler.
- Double-click Prevent Task Run or End, click Enabled, and then click OK.