When you log on over the network there might be a policy that dosnt let you change the computer settings.
Or that your network account wasnt added to the local administrators group so you dont have the access.
To work around the second problem. Logon to the local machine with administrative rights, right click my computer, choose manage, double click on the left side system tools then local users and groups, double click the administrator group and type in the bottom half of the window "workgroup\yourname" where workgroup is the name of the workgroup your user name resides on and username is your user name you use to log on to the network.