Server Notifcation Error

[applehouse]

I am a newbie to Apple computers. I recently purchased a 2011 IMAC top of the line. I love it! I attempted to move files over via my old Sony PC via Cat wires and did it as a shared network. When I attempt to change my wallpaper in my controls I get an error message that the "Name of the old Computer" network cannot be found. This is preventing me to change wallpaper via the control board as well as change the screen saver. It is only happening with this account which happens to be the master account. Does anyone know a way to get this network off my computer. I have tried going into the network section and it is no place to be found..It is all very strange. Any help would be greatly appreciated!