Before you in-install Metrix, create a backup of the database using OSQL.
If you changed the 'sa' account password during the Metrix Server installation or any time after and have since forgotten it, you'll have to uninstall Metrix Server, which should automatically un-install SQL Server 2000 Desktop Edition (aka MSDE).
This is because the administrator's Metrix password is actually the SQL Server/MSDE password. See I cannot uninstall Metrix Server for issues with un-installing Metrix.
Creating a Back up as a Metrix Administrator
Since you can't create a backup in OSQL as the system administrator, i.e., you don't know your 'sa' password, log in to OSQL as a Metrix administrator. Refer to Using OSQL to backup and restore a database