username and password request

Yesterday I began getting a request for my username and password.  After I enter them it pops up again.  I have a second Windows Mail account on this computer and it works.  I have searched through Google for a fix and anything suggested has no effect.  I downloaded Windows Live Mail and the same thing happened.  I tried to install Outlook and the same thing happened.  I contacted ATT and they do not support Windows Mail anymore.  I dont' have any idea if this is an ATT problem, a Windows Mail problem, a Vista problem or if I have a virus.  A lot of information I found said antivirus programs can cause this.  I downloaded Microsoft Security Essentials a week or so ago but my email worked after that.  Does anyone have an idea as to what to try next?

Post Originated from Vista Support Forums