This is a common problem without a simple solution, assuming you're trying to monitor every device from a central location. You can't monitor these kinds of stats from a single PC, you need to get in the middle of everything, your router.
I've been in similar situations with bandwidth-hogging roommates, I was able to flash my router with the Toastman Tomato Firmware which will allow you to monitor and restrict bandwidth usage on a per-device basis (note: the standard tomato firmware won't break down bandwidth usage by individual device.)
If you want to go this direction, the link provided explains the process from flashing the firmware to setting up monitoring.
It is a rather involved process, and you need to make sure your router is compatible.