Nearly always problems like that have to do with firewalls, router settings, virus software etc on your PC
Apple has a KB article on the fine points (what ports to open and other settings)
I'll assume you've done the basics: made sure that the name of your network is broadcast, enabled home sharing in iTunes, iTunes is running.
an aside: IMHO best bet network wise WPA2 for nearly everything)
Anyway, here are some links that may help
one about tshooting Wireless issues:
This one has to do with opening the required ports:
anyway I found both very quickly via Apples ATV support:
worth bookmarking... a more thourough search is likely to help
Lastly, you might even try calling Apple if you're still under warranty..