Way too much internet bandwidth being used?

How do you get the ISP usage data? Can you get the usage over short periods of perhaps one hour?

As an example, if you have an 8 Mbps (Megabit/second) package from your ISP, then that is a maximum of 1 MBps (MegaByte/second). In an hour, the maximum you can use is 3600 MB or 3.6 GB. If the ISP says you have used significantly more, then there is a problem with the way they are measuring your usage.

You say that the usage is zero if you do not use the computers. If the ISP can only give you a daily report (like mine does), download large files continuously for one hour and don't use the computers for the rest of the day. In fact, shut them down so no background processes are using your Internet connection. How much usage does the ISP say you have used? If this exceeds the maximum you could have used in the test time, then its an ISP problem.

If you have a wireless network, change the wireless encryption key or disable the wireless and just use Ethernet connections. This will eliminate freeloaders.

If you are watching a lot of movies and video clips online, avoid using HD. Lower resolution video is acceptable for many films.

I hope some of this helps.

In Linux in ifconfig you can see the cumulative bytes used on each interface. I believe on Windows the same in ipconfig. Probably that's what networx is doing; I'm not familiar with it.

So you need to check the usage on each computer to see which is the culprit, unless you can do flow monitoring on your router.

If you run wireshark it will tell you what the traffic is exactly, then there are tools to tell you which application is responsible. It can get complicated.

If you have certain malware that is doing stuff on the network like sending spam or DDOS, that would explain it. Or it may just be stupid websites. I've seen weather.com consume I think 100Mb overnight with no-one using it, just because it does page refreshes and shows a load of adverts.

I'm not sure if it's still the case, but if you have Skype running on a direct internet connection for more than a day it can start forwarding traffic for other users. Not usually a problem for home connections with a NAT router.

At a crude level, I think flashing LEDs on the router interfaces indicates traffic, so if the lights aren't flashing that connection is probably not involved.

I'd start by leaving the modem connected and disconnecting your computers. If you computer isn't on the network it can't use bandwidth. That will tell you definitively whether or not it's an issue specific to your computer or is it's something else (like someone downloading large files on your wireless connection without your permission).