1- Check if you have ms sql server jdbc  drivers in your path (in WEB-INF/lib)
2- MS SQL server  2000 has two modes for authentication: windows and sql server authentication. Choose sql server authentication in your sql server configuration (from Enterprise Manager).
3- Check if the user you are using to connect to database has enough permission for example to read or change database tables.
4- Check your database URL for correct host, port, and other parameters.
5- And never get hopeless, you'll find the answer