I have noticed that many people can have issues for various reasons when it comes to there network.
Make sure that you understand all the router requirements for each different system/software you are trying to use.
Media Center, Xbox and PS3 can have different MTU requirements, different UPnP requirements etc.
Sometimes you will have to isolate each device in your network to get everything to run smoothly. This means the Xbox will be on the primary network with your PC on a secondary network. I have had to use this setup to aleviate network issues and have had no problems going forward. Different routers, different scenarios, so you have to find something that will work for you.
Interference is a big issue with wireless networks so be prepared to try a direct connection to get this to work. If that means moving your Xbox/tv, or your router, so be it. We cannot help if you refuse to do this. It is a necessary required step to elimate any wireless issues(or to confirm wireless issues).