I don't know if you can disable the prompt in Explorer, but there are file manager apps that can copy selected files with a button push and you can set the option to prompt or just overwrite without asking. For example, FreeCommander has F5 for copy and F6 for move, the default is to prompt before overwriting, but you can change it in the options. You can even set it to use somne 3rd party copy apps such as TeraCopy instead of Windows shell.
The main drawback with FreeCommander is it doesn't come in a native 64 bit implementation. Even so I get use out of it on Vista64 and it works fine on Windows 7 32 bit.