Best / easiest free bandwidth or usage monitoring software ?

[#1]

I've just had a call from an old (in both senses) customer - he's on a mobile dongle and is using his 3Gb allowance rather rapidly.
He's called O2 who have told him to install some software to monitor his usage but he turned off at the word "install"
Anything out there which would log what is using his allowance up and is pretty simple to use ?
Ta
Dom

[#2]

Assuming Windows is the OS, I've seen networx recommended on these forums.

[#3]

If he doesn't want to install anything, does his existing firewall report usage ?
When I'm using my mobile dongle, I always keep my Firestarter firewall window open to monitor usage.

It is also very useful to see which programs are connecting to the internet !

[#4]

Thanks for the replies guys - he's  pretty green when it comes to computers so I don't want to confuddle him. I also need something I can use here so I can talk him through each step over the phone if I need to.
Just having a play with Networx and it seems reasonably easy to use.