First of all, I assume you're talking about a
wireless network, right?
I agree with the previous post about starting from the simplest possible configuration and adding in features one by one. This is the easiest way to isolate the problem. Also, here are a few links to sites/forums with good info on wireless networking issues, etc:
<ul type="square">
Notebook Review networking & wireless forum
Practically Networked wireless troubleshooting page[/list]