If you are using Windows XP you will have to disable the built in firewall. But for all operating systems you have to disable any other form of firewall software such as Norton Internet Security, Black Ice, and ZoneAlarm.
Make sure that you have "Client Services for Netware" installed, it should automatically install IPX/SPX. IF not please do so.
Note: Windows XP Home edition does not have the capability to install Client Services for Netware.