Hello,
I would do it like this (might be a better way)
1. Install SQL2000 on the machine with WSS.
2. Make a full/complete backup of existing SQl databases
3. Restore the databases onto the WSS machine.
4. Go to SharePoint Central Administration and change the location of Default Configuration Databse and Default Content database.
Just make sure that you leave the "old" database server untouched until you are sure that your sites work properly with your new SQL database.
However having the WSS and SQL on the same machine is not a solution I would recommend since it will have a large impact on the performance of the server. SQL can become very memory intese and also WSS takes some amount of memory depending on how many virtual servers and sites you have on it.
Regards,
Thomas