The next thing I would do is completely disable all of your security features temporarily. Also, if you are trying to do this wirelessly then I recommend running a cable instead just to see if that makes a difference. You also might want to switch to default settings on your 360. You can also read about different alternatives to WMP11 and see if they work.
Do you have a second computer in your house? Are you able to connect to the 360 using that computer?
If all else fails, you can also switch from XP Pro to either XP Media Center, Vista Home Premium, or Vista Ultimate. I am not sure if that will make a difference but I have read stories about users who just couldn't get it to work until they made a switch like this. The 360 just seems to work better as a media center when a windows media center OS is being used.