You may change encryption level on terminal services in Windows 2008 using Group Policy. Use these steps:
Open the Group Policy applet.
Navigate through Computer Configuration --> Administrative Templates --> Windows Components --> Terminal Services.
Select Encryption and Security.
In the right pane, double-click the Set Client Connection Encryption Level setting, and then click Enabled.
In the Encryption Level list, click the desired security level.
Click OK to finish the procedure.