I need to know a few more things in order to help: First - forgetting all the wireless devices - when your PC is connected via the ethernet cable - with wireless disabled - can you in-fact get out to the internet - from internet explorer - and browse web pages? If you can do this - your ethernet adapter is probably working fine - and you can safely assume the problem is not the result of a defective ethernet adapter.
Secondly - the 255.255.255.0 is the subnet mask - and not the actual subnet. The subnet mask is used in conjunction with the ip address to determine what part of the ip address is the subnet - and which part is the host address. Example: if your ip address (that you see with ipconfig) is 192.168.1.132 - your subnet id is 192.168.1. All of your devices on your network need to have the same subnet mask of 255.255.255.0 and also an ip address that starts with 192.168.1.x (depending on which router you have you may see ip addresses that look like 192.168.0.x - as long as all your devices (wired and wireless) start with the same 3 sets of numbers - those devices will be on the same subnet and can see each other.
Can you look at the apple tv network settings - and tell me what both the ip address and subnet mask are - as well as going to ipconfig on the PC (with the ethernet cable plugged in and wireless disabled) to get the ip and subnet. Don't worry - these are internal ip addresses and not accessible to the outside world - so you are not compromising your network by indicating those here.
If any of your ip addresses (on your are starting with something other than 192.168.x.x 10.x.x.x 172.x.x.x or 128.x.x.x then your device is not using an internal LAN ip address and that could be causing you a problem.
Once you determine the ip addresses of both the apple tv and your PC (with only the ethernet cable) - open a command prompt on the pc and type the following command: (for this example I am making up an ip address for your apple tv of 192.168.1.103)
ping 192.168.1.103 <--- you would substitute your apple tv's actual ip address in place of 192.168.1.103
you should get at least 4 responses indicating:
xxxxxx bytes from 192.168.1.103
if you get a response indicating "request timed out" - then your PC cannot see the apple tv and we need to figure out why.
One other thing to check - is there any possibility you have the windows firewall or some other type of firewall software that is maybe locking down your ethernet adapter but not the wireless adapter?
I am curious to know what you find.