nvmctray.dll is a module installed alongside NVIDIA's range of graphics cards. The System Tray icon is used to manage settings for nVidia based graphics cards.
The process may be required for some 3D applications to recognize your card correctly - such as the game "Everquest". Otherwise, settings can be changed manually via Display Properties.
Non-system processes like nvmediacenter originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that your registry has suffered fragmentation and accumulated invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues