The Unix side does this with "nice" and "renice". Windows may have something like it.
Did you log on as administrator on that machine?
Do you have to stop the process before you can change its properties?
(MS Windows 2000 online help isn't much help...)
If you schedule the task manager to run by using the at command, it will then run under the local system account. As for permanently changing the process priority, that would be done at the code level of the application or service.