Internet Usage Gone Amok!

Question

Hi all (and Tesserax if you're online, you know the history of my problem!). I am posting this here because I can't find another forum which seems appropriate.
Short version: for two weeks we've been aware that our internet usage has been exceeding our use. We have two iMacs networked in a small office, both are running Snow Leopard.
We have tried eliminating what we thought the issue was - namely that someone had hijacked our wireless somehow.  So: we installed a new D-Link router (which is WPA encrypted AND then the entire wireless radio disabled), and we have a new cable modem from our ISP, along with a new virus program (Intego Virus Barrier X6). We scanned both computers and Intego found one virus (a file called 1024.dmg which had been on one of our computer's hard drives since 2008!) and one attempted port scan, which was blocked. Other than that, there has been no suspicious activity reported.
I called an Intego technician who assured me that their software would catch anything out of the ordinary and so therefore it can't be anything on our computers. The Intego guy said that only a trojan would be capable of doing something like this, but again, VirusBarrier would catch that. Also Interesting to note: it's the downloads that are extraordinary, while the uploads are consistent with our day to day business.
Even with all of these things up and running, we're still experiencing spikes in our internet use that has us and our ISP puzzled. Two days in the past month we weren't even in the office at all and there were 6 gigabyte spikes on record!  However, our computers were on, but in deep sleep mode.  Something fishy...could someone be hacking in even with our computers in sleep mode????
My ISP mentioned that they have a client who's son let their bit torrent open and someone was downloading off of their internet access, running up the father's bill to $900. From what I understand, bit torrent is a P2P protocol. We don't do ANY file sharing that isn't between the two computers on our LAN.
is there any way that someone can get in that way?  On my computer, I have file sharing, printer sharing and scanner sharing enabled.
Oh - and we thought that maybe our ISP's usage tracking was off, so for three days we unplugged our ethernet cable to test. Then we checked the usage for those three days and there was nothing.  So something is definitely happening on our end, but we're beside ourselves trying to figure this out! I am wondering if anyone can assit me here.  Tesserax has been very kind in offering suggestions - if you have any more, I'd appreciate it!
thanks
B.

Answer 1

Hi Tesserax.
Already did it: the numbers are a combined total from both computers.
Further to me posting that last note, I did some online research and found scores of complaints on different internet forums commenting on the exact same problem I've had AND with the same ISP.  There is a general consensus that their internet meter is faulty.
AND: I found articles where Bell has actually admitted to having faulty metering.  So this is nothing new.  What irks me is that the earliest post I found is from 2009 and no one at my ISP even mentioned that this could be the issue!  Nor does it seem that they're in any hurry to rectify the problem.
In some cases, their numbers are astronomical - 56gb per day even!
At least it's not on my end and that we don't have a hacker, which was my real concern.
But worth posting in case others come across the same issue and think, like me, that someone is stealing their wireless.
Anyway, thanks again for all your help. 
B.