How are the PC and ATV2 connected to your home network? Many routers block two WiFi devices from talking to each other. If you know the IP address of the ATV2 it might be worth seeing if you can ‘ping’ the ATV from the PC is a Command window.
Do you mean that the ping is failing, or that you have not worked out how to do it?
If it is failing then it suggests you ahve a network issue. I would look closely at the settings on your router to see if there are any that relate to devices on the same WiFi network talking to each other.
Another possibility is to see of you can do AirPlay to your ATV, or if the ATV can play items from iTunes. In fact in the past some users reported success trying to install if at the same time they had something using AirPlay.