outlook problem

hi all
when i am trying to adjust the pop setting of my yahoo mail so i can recieve my mails on outlook , i got a problem…each time i try to recieve my mail i got a message asking me for user name and password even they are set before and conitually show me this box even i write the password again and again. i tried to alter the security and authentication settings but all fails..
any could help me
thank u all very much