I'm guessing by the access denied error, you're not using a Domain admin account. So to access the other PC remotely for management you are going to need to add your user account from the domain to the admin group on the other PC. Unfortunately I can't give you much help on the exact steps as I don't have Win8 lying around at the moment. Assuming it's User wizards are similar to Win7 you will need to choose the option "Give other users access to this computer" and then add your user account from the domain as an admin user. It's best to put the username in the domain format, e.g. MYDOMAIN\Username when you do this.
Once you're a local admin on the other PC, you will have the access required to remotely administer the PC.
BTW all this is assuming you don't want to use RDP or something similar to just remote in and manage the PC.