Several weeks ago, I entered this conversation regarding "invalid password" error using WLM and ATT/Yahoo Mail on the Internet. ATT seems to have admitted there might be a problem, but I never saw anything to confirm that, or a fix from ATT/Yahoo.
The advice given in the thread then was that my new iPhone was causing the problem, due to the setting "push" being "on" and whenever the iPhone is awake, it keeps logging into my ATT/Yahoo mail account. Since POP servers do not like 2 simultaneous users for one account, my PC got the error. I leave my iPhone on all the time, on its charger.
Well, I'm here to report that all I did was turn off "push" on the iPhone, and the problem has totally vanished! All I have to do now is "refresh" on the iPhone and my emails pop up.
Thanks to iFiredog and Mike Distefano who offered that advice and information!
Thanks again!