Anyone got any other ideas?
Install
Network Meter on all your PC's. Watch the upload bandwidth.
Just captured it on my PC to show ya how it works:
At the bottom of the window, the "total" numbers are since last reboot, the "current" numbers are current totals for the month. Green is upload, yellow is download. You can adjust the monthly resets to sync with your ISP's billing cycle.
Also run combofix on your computers, to look for rootkits. Normal virus and malware scans won't detect them.
What router are you using? If you can flash it with DD-WRT it has advanced bandwidth logging on a per computer/phone/tablet basis, really great for seeing WTF is going on with your data usage!! I had to do this to figure out WTF was going on with my network, you wouldn't believe what it was, MY CANON T2i DSLR CAMERA!! lol. I had one of those EyeFi wireless SD cards in it, had cloud sync turned on, and it kept trying to upload a 700mb video that would fail, and it would just continuously retry to upload it over and over!!! yikes!
Are you with Rogers? If they are dinging you with overage charges you can upgrade to unlimited for $10 more per month.
Good luck.