You are going to have to set up a policy within the Group Policy Options to have it run the Defragger. Since the users dont have Administrator rights, I am guessing that this is on a corporate network. Which means they are part of some sort of Group Policy that regulates what these users can and cant do.
You are going to have to create a group or policy that will allow either the users to sign on or create a task within the network to run the defragger on a schedule over the network.
Sorry i cant be more specific, but this area of the site is for home based users. Our area of expertise is not a workstation on a company network. I can move the topic there, but as far as getting directions on how to specifically do it, you wont get that in this area of the site. That is part of our Enterprise Area that focuses on businesses and their questions.