Ethernet is an 8 wire twisted pair system. In fact only 4 of the 8 wires are used. 2 for data transmission and 2 for handshaking. So you will need a minimum of 4 wires for Ethernet.
The alternative is either Wireless or Homeplug. WiFi uses wireless and HomePlug transmits the data and handshaking over the 240v mains cable.
Coax is just that. 2 wires coaxially paired. It's designed for relatively low voltages and to carry RF signals, mainly from antennas but also between some items like VHS and TV. However it gives minimum quality and people use SCART or HDMI for better results between components.
My guess is trying to mix the two systems would cause interference to both signals.
Window media centre is rubbish, don't even think if using it!
If you pipe sky around to more than two tv's on one sky box you will end up with a rubbish picture (and you all have to watch same channel) and have problems changing channels, will need a remote extender.
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