Monitoring internet traffic

At home we have a DSL connection with a limited MB traffic per month. My
computer is connected to the router and my wife's has a usb adapter for
wireless access.
I would like to monitor how many MBs are being downloaded from the internet
for both computers so that we know if we are getting close to our limit.
I tried a couple of free trial software out there but the problem is that
they also add the MBs of data transferred from one computer to the other. I
only want to know how much is being downloaded from the internet alone.

Anyone know of a program or two that I could choose from?

My router and Adapter are from Siemens. I tried installing the "PRTG Traffic
Grapher 4" from paessler.com but I am not sure if my router supports SNMP (I
searched the router's user's manual for snmp and got no hits). I am new to
the whole networking topic and don't know my way around too well. Any
suggestions are appreciated.

Thanks.