Could be you have something that activates and tracks "wow.exe"?

A friend is using a Logitech G15, as soon as he started up a 2nd version of WoW it would crash and cause strange bugs, because the G15 tracks and displays info given to it by the application, I'm guessing more than 1 copy running sends it twice the info, and confuses it, and locked up his system. I'd try renaming the .exes to something else if you haven't already