Have you setup your Windows 7 computer for file sharing and network discovery to 'on'?
You need a media sharing software. I use Plex for my home network media sharing but there are other software available. Have you tried to configure Windows Media player instead (if you don't want to install additional software)?
I don't know if you already used the same guide that I found.
http://www.directv.com/viivDownloads/pdf/DTV_quickStartGuideNewVersionFinal2.pdf