I've done this for years, although it is a halved for a PC and XBox - they both won't be maxing out their bandwidth at the same time. I don't think there is any problems of interference - the 4 pairs are twisted for the full length of the cable, and seperated at the double wall socket at either end. Just the same as gigabit ethernet uses, except the data is not combined into one line.
People, this 100 Megabit thing is a Red Hearing. You don't need to worry about 'combined bandwidth'. If that was the case, you wouldn't be able to Transmit at 100 Megabit while Receiving on the other differential pair at 100 Megabit rates.
If you don't want a switch (and it's latency) between you and your ISP, this will work just fine.