Do you want to run a scheduled task that will change a user's password, or change the password of a user that is running a scheduled task and have the scheduled task stay valid?
Do you want to do this using a BladeLogic job or with MS Windows?
You may wish to try using the change parameter of the schtasks command. http://support.microsoft.com/kb/814596
You may also wish to setup a non-expiring service account for scheduled tasks in the future such that passwords need not be changed (or have extended change intervals).