While both the Xbox 360 and the Ps3 have the built-in capability to access your home network, it's sometimes quite a pain to get everything configured properly with the right permissions so that everyone can share properly. Also there is a ton of video formats that both the PS3 and the Xbox 360 can't understand.
Instead of messing around with network settings and video converters the easiest way we've found to get the movies on your computer to play on your console hooked to your TV is by using PS3 Media Server.
Download PS3 Media Server here : http://ps3mediaserver.blogspot.com/ (Click on the latest version and then click on the .exe file for Windows)
Once you install this software simply double-click the icon it puts on your desktop and do the following:
1.Click on Navigation/Share settings tab
2.Click on the big green + symbol
3.Find the folder that contains the video files you want to stream to your console.
4.Click 'Ok' and that directory path should be all that shows up under Shared Folders. (Delete any directories you don't want to share)
5.Quit and Re-open PS3 Media Server
Once it's running on your PC turn on your console and go to the Video Library on the Xbox 360 or 'Videos' on the PS3. You'll see an option for PS3 Media Server and can access all your videos through your TV. Also if the file on your PC is not compatible with the 360 or PS3 it will convert it on the fly to the right format.
Keep in mind that for High-Def movies you will need Wireless N speeds to have enough bandwidth over the network, for best results use physical network cables to connect the PC to your router and the Consoles to your router. DVD Quality movies are fine.