Which password is it accepting when this prompt comes up? Your windows login or you email authentication? If it is accepting your windows login make sure you don't have an exchange account set up that you aren't using . Also did it ask you just the once, when you reset your windows password or is it every time you restart or log out that the prompt returns when you open outlook? I don't think the domain server will force you to re-enter your outlook info by default or any regular setting, as I cannot recreate your situation but I know there are scripts that can be run to force OWA to prompt user for password change which could also do this to your outlook.

http://www.atot.net/microsoft-exchan...word-using-owa

I guess it depends on how you changed your windows password, was it something the domain forced you to do after a set time or did your IT go in and say user must changed password on next login?