Disabling re-authentication in WinXP machine

Have a combination of Win98 and WinXP clients within a WinServer 2003 domain. After some idle period (1 hour or so), users on XP machine will be prompted to enter their username and password before they are allowed to access the network again even though the machine has never been logoff. Doesn’t happen to Win98 machine. Any idea on how to allow WinXP users to continue using their machine without having to re-enter their credential again or disabling their machine from prompting them for username and password after certain period of idle time? Have tried to disable both auto-logoff and user idle timeout in the Default Domain Policy but nothing seems to work. Thanks.