Just for clarity - what are you trying to use Home Sharing for on the PC?
Compatible content stored in iTunes would be accessed under the AppleTVs Computers icon which will list any detected Home Shared itunes libraries on the LAN - if none it will moan that you need to set up Home Sharing.
As Remote app and mirroring work from iPad it would suggest that the primary issue is communication with the PC.
Again for clarity, iOS devices either do lossy compressed screen sharing (Airplay mirroring) if new enough OR can send media files directly to the AppleTV to decode/playback (stored music and video on teh iOS device - a stream is pushed to AppleTV) - photos may work slightly differently, but do you see an Airplay icon in the photo app? AppleTV cannot 'pull' material from an iOS device using it as a source, things are 'pushed' from teh iOS device.
With computers, Home Sharing iTunes allows you to use the computer as a media source and pull audio/video streams from iTunes. To confuse matters items can also be pushed to AppleTV from itunes using the destination 'speaker' icon which now resembles Airplay. Windows PCs and most Macs over 12-18 months old cannot do Airplay mirroring of screen content, but there's a 3rd party app called AirParrot which can do this though performance may not be optimal in some cases.
AC