Is this machine running Windows XP Home edition? Oracle software is not supported on Windows XP Home edition. The most common symptom of running Oracle on Windows XP Home edition is the ORA-12571 error. The only fix to this problem is to upgrade the machine to Windows XP Professional.
If you are running Windows XP Professional already, then I would suggest uninstalling and reinstalling the Oracle software. There are too many things that could be wrong here and too many places to look. It is easier to reinstall the software than to track down this problem.
This was first published in September 2004