In the absence of further info, a few assumptions :
1. IE or some variant.
2. Windoze or some variant.
3. You've blatted cookies, temp, files and the rest.
4. The "remember me" tick box is deffo unticked.
Foremost possibility in my mind then is IE's auto-complete feature.
When you go to the web page, do you double click in the "User name" box, and it remembers the password ?
If so, double click, highlight the username, then press Delete. Should work.
If my assumptions are wrong, ignore the above.