----- IF We're Talking About A Windows Password -----
Not sure what you've tried, but do the following:
1) Do a search for *.pwl files and delete all results
2) Go to Start->Run, type sysedit, and hit OK. In the System.ini window, look for the section [password lists] and remove all the lines below that header in that section.
3) Go to Start->Settings->Control Panel. Double-click Passwords and go to the tab "User Profiles". Make sure "All users share the same preferences..." is selected.
4) Reboot and if prompted for a password, leave it blank and hit OK. This should be the last time you're asked for a Windows password.
~cdogg