Follow the steps below to create service account(s) for SQL Server 2000:

1. Create Logins for the SQL Server and SQL Agent services. Could either be be one account for both, or separate accounts for each. Make you checkmark password never expire and logon as service.

2. Add this account(s) as SQL Server Login and grant it SysAdmin rights.

3. Configure the services to use these accounts. Use Enterprise Manager to do this; it will automatically set all the necessary server and registry settings.

4. Stop and restart the services, test to make sure everything is working.

Link: SQL Server 2005 Service Accounts Configuration

Link: SQL Server 2008 Service Accounts Configuration