can't surf net and use xbox at the same time

I'm having touble getting the mn-700 to share my internet
connection with my pc and my xbox. I have the xbox using
the mn-740 and have my pc direct connected to the mn-700.
I can connect to each one seperatly, but not together. If
xbox is running, internet explorer won't work and vice
versa. I've tried everything, the only thing that might be
a little different is that I have a USB ethernet adapter
instead of a NIC card. I bridged my dsl modem (Zyxel 645m)
and have the m-700 using PPoE. I use earthlink and they
support multiple internet connections. Please help.