You should be using GPO for this. There are other methods but they are not as secure.

http://technet.microsoft.com/en-us/library/ee390958.aspx

http://www.chicagotech.net/netforums/viewtopic.php?t=447

You can also search the board for Runnas (notice there are 2 n's) written by one of the old time Moderators here named Shawn Tassie (this is not the recommended way though, using GPO is the recommended and appropriate way)

Good luck