Hi, I short you will need a database or other persistence for holding the users' ids and password for validating against.
Next, you can create your webservice in the language of your choice but I will use ruby in my explaination. This webservice will either communicate to your persistent entities using Rails AR or the Ruby gdbm module.
Now, you will create a standalone SOAP server that regsiters your webservice. See Page 249 of "Programming Ruby" for more detail.
Finally, you should be able to refactor existing authetication schemes to easily use your web service.
Good luck,
-Cconrad