When your world of Warcraft logs in it starts a patchy download that is never successful how do you fix it

The easiest way would be to download the patch that went wrong again, from one of the mirror servers (check wowwiki, they have some listed) and then run it to apply it to the game.

If this doesn't fix it, another option is to run the repair utility that comes with World of Warcraft; it should be in the game folder and be called repair.exe. Keep a backup of your WTF, Cache and Interface folders, because repairing might wipe them clean.

If all else fails, you can always reinstall the game, but that will be a time-consuming process.