I once had the problem that I changed some settings in windows which disabled my automatic login into windows xp. I was prompted to enter my login details and could not remember them at all. The following method could be working in this and similar cases. You could try using the default Administrator login and leave the password blank. This should work for every computer where the admin password has not been changed.
You might need to boot into safe mode if the target system does not use the welcome screen of windows xp or if you can´t log off. (Try hitting CTRL + ALT + DEL twice) Boot into safe mode by hitting F8 at the boot process and select Boot Windows in Safe Mode. The following welcome screen will display the Adminstrator icon next to others. If you have not changed the password you will be able to login by leaving the password empty. This way you are logged in as administrator and will be able to change the passwords of the other users.
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