This happened twice yesterday to one user on my systems here at work. I did not see it in person because I only get called to fix the problem so the only things I can tell you are what he told me before I solved the problem. In the morning he came into work and logged in as usual, upon loggin in his password expiration notification came up saying he had 14 days left. He cllicked no to changing the password deciding he would do it later. A few hours later he went to lunch and when he came back his screensaver required him to log in which he did. Got back into windows no problem but when he tried goin on the intra or internet he got the username/password/domain entry box. Tried entering his usual information but wouldnt let him on so thats when he called me.
I looked at it and figured he was locked out for some reason (I thought mistyped password x times) so I went and unlocked his account for him. Fast forward to five oclock that day and I leave for the night. About 30 minutes I guess he has the same problem again, walked away from the computer for 15 minutes or however long until his computer locks itself and when he logs back in no intra or internet available same username/password/domain box appears.