I would suggest as a newbie (if you haven't already) get the Nagios book by Wolfgang Barth. I have learned alot from it.
As for your specific problem. I would need more details. Like how are you communicating to your systems? Are you using SNMP checks and the WMI interface or are you using a client like NSClient++? Are there any firewalls in place between your Nagios box and the other systems. (the list goes on....)
If you can better describe the error you are getting, I or someone else may be able to help point you in the right direction.