Instructions
Click the "Start" button, then type "World of Warcraft" into the "Search" dialogue box.
Double-click the "World of Warcraft" directory folder in the listed search results.
Double-click the "Data" folder.
Right-click then delete the "Patch.MPQ," "Patch-2.MPQ" and "Patch-3.MPQ" files.
Click the "Start" button, mouse over "All Programs," mouse over the "World of Warcraft" directory, then click "World of Warcraft-Repair."
Click "Reset and Check Files."
Launch "World of Warcraft" once the repair utility has completed the repair process. The auto-updater will re-download and apply any necessary patch files.