WSS 2.0 does not synchronise itself with Active Directory. This means that if a user account is renamed or has been moved to another domain, it can no longer log in to SharePoint. You can of course add the new account to SharePoint to grant access but then you end up with two accounts representing the same user which is not ideal for clarity in history, etc.
The correct solution is to update the username across all the content databases. With Service Pack 2 a new API was introduced and with that a new operation for stsadm.exe called 'migrateuser' which does exactly that.
For example, if a user account has been moved to a new domain, it can be migrated with the following command:
stsadm -o migrateuser -oldlogin domain1\joebloggs