I have a Linksys WRT54G2 router (slim black design, not the older WRT54 blue model). I have a XBox Slim which has an integrated wireless adapter (not the older XBox 360 with which you had to purchase the wireless adapter separately).
I have Linksys router connected to my broadband modem.
I have my desktop PC connected via Ethernet cable to port 1 on the router.
I have my XBox 360 connected via Ethernet cable to port 2 on the router.
My wife's laptop and the television utilize the wireless internet.
Now, if I use my desktop computer, then turn it off, and then try and connect my XBox 360 to XBox Live, I can't get a connection.
When I go to the Dashboard and view the network, it constantly switches between "wired" and "wireless." It acts like it can't choose which one to use. I can't disable the wireless network adapter on the XBox 360 because it switches too fast and it simply doesn't give me the option (I think it would be to press the yellow button on the controller).
Anyway, I can connect to XBox Live if I connect directly to the modem.
I can also connect if I reset both the modem and router. Sometimes this requires a few attempts.
But, why is this happening? I don't really understand it. I did hear something about MS having some sort of proprietary software that causes a conflict when using the XBox 360 on a wired and wireless network. But, I don't know about the validity of that statement.