Are your wireless and wired networks on the same subnet? I don't think it matters, but it might.
I'd try the following either way:
Run a command prompt and do "route print". My guess is you'll see two default routes (Network Destination of 0.0.0.0, Netmask 0.0.0.0). The Interface column should list the local IPs that are used for each of these routes. If the metric for the IP on your LAN is a lower number than the one for the WLAN, your traffic is being sent over the LAN.
If it's going over the WLAN instead, you are probably best off just turning Wireless off. I think there are utilities that will do this automatically when connected by wire.
If you really want to leave both running, you can set metrics on the interface. Under the network adapters properties window, networking tab, open IPv4 properties, hit advanced, and down at the bottom, you can deselect interface metric and put in a number. Once again, you want your LAN metric to be lower than the WLAN. Do this for both, then do a "route print" again to see that it looks ok. Like I said before, though, I probably wouldn't pursue it in this way.