Move the router or get a longer cable are your only too main options,
short of buying the wireless adapter, or move the XB360.
If you have spare routers in your house somewhere, you could attempt
to create a bridge network with them, but this would most likely fail,
or seriously slow your connection speed by the time your XB360 receives
the signal.
With my set-up, my router and hard-ware is in my room, but the phone-
socket it plugs into is downstairs, so I checked the blueprints for my
house, checked under a floorboard to see what was beneath it, then
drilled a small hole in the corner of my room, and fed the cable down
there, then clipped it to the wall downstairs and led it to the socket.
See if you can do the same, just be careful of water pipes and electric
cables, and if possible, turn them both off before you proceed, (check
your house blueprints too), this should shave a few metres off the total
length of cable needed.
