After trying everything else through Battle.net and going to youtube, search engines, firewalls, security protection turned off(5 of these), safe mode, apps closed, secondary user account changed, client supported adds deleted, proxys ect.; I searched 'wow patch 5.0.4 cannot use files' and found this thread. Having originally transferred WoW files from another comp years ago and had been sitting in a 'windows.old' folder. I cut and pasted as suggested in above post and it worked immediately. Now the launcher is slow, 1% download after 10 mins. We'll see what happens next. Thanks for the info!