Ethernet Issue - Help Required

Complte noob to this. Am far from tech savvy so bear with Me.

I’ve sucessfully flashed ATV, have hooked up My PC to it to stream using an ethernet all with not to much trouble. Problem is one I cant figure out how to keep the internet connected for for online streaming at the same time.Once I plug the ethernet in it goes to DHCP and I loose My internet connection on ATV. I can uplug it and it picks the wireless up straight away. I can plug ATV directly into the wireless router again with no problem. What I want to do is to be able to stream from My PC AND be able to use Youtube/Boxee etc on ATV without having to unplug the network cable all the time. I guess if its possible at all it has to be some kind of manual IP configuration??? Could anybody take the time for a walkthrough on if and how this is possible??? Would really really appreciate it.

If its already been covered and I missed it, apologies in advance.

Thanks guys