World of Warcraft is a beast to install alone, patching it up to the latest version is another story. Many people, me included, will make a backup of the World of Warcraft folder to save time installing it on a new machine. This used to work just dandy, but now you might find yourself presented with an error message that says:
Well fear no more, the fix is easy and takes about two seconds. Go into your World of Warcraft folder and double-click repair.exe. When it opens, select reset only. Re-launch World of Warcraft and your patch should go off without a hitch.