Virgin Media IP addresses are pretty sticky, in my area, it is next to impossible to change it. But, call VM, ask them to change it and they will, because you are ddosed....
Saying that, I believe that you are having problems, because they detect your xbox's MAC address and they disconnect you. You can solve this
http://www.se7ensins.com/forums/thre...dress.1189189/
I am not an xbox expert. So somebody else might come and offer different/better solution.