If you're running Windows Vista, try starting up WoW by right-clicking on the World of Warcraft shortcut, and choosing "Run as administrator" ([ame]http://i130.photobucket.com/albums/p276/bitsf/vista_admin_02.png[/ame]).
I also got the fail to apply patch error. C:/WorldofWarcraft/commonfiles/unitistall.xml or something along those lines.
Used "run as administrator" on my Vista and this solved the problem. Thank You.
you'll need to double-check to see if your User Account in XP is in the Administrators group in User Accounts in Control Panel.
Otherwise, try each of the steps here:
http://us.blizzard.com/support/article.xml?articleId=21066
http://us.blizzard.com/support/article.xml?articleId=21068
Method 2
If the first method didn’t work, follow those steps:
Shut down the computer.
Unplug the modem.
Unplug the router.
Wait about 60 seconds, then plug your modem in and let it turn on.
Plug your router in and let it turn on.
Turn on your computer and try to play the game.
The problems seem to be caused by the WOW Launcher trying to download a small file, which takes much longer than the estimate time of 2 minutes. The download either fails or takes too long. After that, users get an “unable to validate game version” error.