Hi there,

Can you please try to restart into Safe Mode with Networking and see if this helps:
http://eu.battle.net/support/en/article/how-do-i-boot-the-computer-in-safe-mode

Also, try going to your World of Warcraft folder and remove the Updates directory, then test the game again.

If none of this works, you may be able to work around this by performing the following:

1. Navigate to your World of Warcraft folder
2. Run the Repair program
3. Run the program BackgroundDownloader (as an Administrator on Vista and Windows 7) for around 30 seconds
4. Run the Launcher (as an Administrator)

This may initiate a large download, however after this download has completed the game should work again.

Instead, you can try to reinstall the game. At the moment the quickest way to reinstall would be to use the Cataclysm DVD if available.

Please see if this helps.