Just say "No to all"

When copying a large amount of files from one location to another in Windows Explorer, if some version of the files already exist in the destination, Explorer politely asks if you want to replace the existing files. That's nice, but the choices available in the dialog box are "Yes," "Yes to all," "No" and "Cancel."

What about "No to all"? Say I just want to copy the files that don't already exist, and I don't want to have to click "No" for every file that already exists?

This collection of Windows Explorer Hacks lets us in on the secret: hold down the Shift key and click "No," which actually means "No to all." Not exactly what we'd call intuitive, but good to know for the frequent file movers and copiers among us.
Windows Explorer Hacks [windowsdevcenter.com]