It's a Catch-22. For this to work, you must logon as that user. But Access by default always logs on as Admin. So (without using vba code) you need to create a shortcut that logs on as the user you want to change. The Target of the shortcut should be:
"C:\program files blah, blah\office\blah\msaccess.exe" /user "someuser"
DON'T try to use the ridiculous 'fake shortcuts' that most new versions of Access put on the Start\programs bar for Win XP. These are fake shortcuts, you can't edit them. You need to go into the c:\progra--blah blah whatever\ location where access is and create a shortcut manually to msaccess.exe
Then this opens access logged on as that user (with the default blank password) and you can now change the password.
The next time you click that shortcut, it will prompt you for the password.
--Jim