Malware and P2P use.
You don't mention what anti-malware you use, so I would suggest you perform a full check with your anti-virus scanner, then download, install, run and update, then scan with the following two anti-malware scanners;
Malwarebytes' Anti-malware, and SUPERAntispyware.
You mention "We", so are you sure no-one else has installed any P2P, (Peer to Peer), software like Limewire, or are using file sharing torrent sites like bitTorrent or uTorrent?
Last couple of days have had some huge levels of downloads - our ISP informs us we have use nearly 8G in two days - where as in the last 12 months we have never gone beyond 25G in a whole month. We can't figure it out if there is something wrong at our end or with what the ISP is charging us. We have only been using the system as per normal with visits to facebook, emails and MSN Messenger... Even turning the modem/wifi off at the power to make sure.... No time spent on gaming sites or downloading movies or upgrades etc. Any suggestions? Our ISP couldn't or should I say wouldn't help us.