XBOX 360 wired to PC, then PC wirless to router

Hi

I have a wireless router in the main house, and in another building on
the other side of the garden I have a pc with a wireless usb adaptor
which works perfectly on this network. Next to this PC, I have an xbox
360 also with a separate wireless adaptor on it which connects it to
the same wireless router over in the main house as the PC connects to.
The media extender also works fine between the pc and xbox via this
wireless router.

Now due to the distance between my pc/xbox and the router in the house,
the reception isn't perfect, but still pretty good. When loading up
pictures etc on the xbox FROM the pc VIA the extender facility, the
data is obviously being bounced over to the router in the house and all
the way back again - even though my pc and xbox are sat next to each
other.

I figured that it may be better to wire the pc and xbox together with
an ethernet cable, so media data transfers to the xbox more quickely
whilst still maintaining the wireless conection from PC to router, but
when I make this wired connection, I get faster media extender resonse,
but completely lose the wireless connection from the PC to the router
back over in the main house, resulting in no internet access at all. It
seems I can only have one connection at a time: either just PC to XBOX
(WIRED), or PC to ROUTER (WIRELESS).

Why is this? I have a feeling it is to do with IP address maybe? Or
maybe I can only have one network connection at a time on the PC?

Help!