About a month ago, I started getting messages from the network admin that my connection was being slowed because I was exceeding the daily transfer quota. Now I've been on this same connection for years and have done nothing new lately: I'm not a gamer, don't P2P fileshare, don't host any websites and don't even IM. Nothing to account for huge data transfer. I do work from home, but these messages are usually appearing on the weekends, not the weekdays when I work (and besides my work doesn't require that amount of data transfer). This was the first time I had ever gotten such messages.
I called the admin to monitor my traffic and he immediately suspected a trojan. Apparently everything looks normal until every five hours, on schedule, there is some program on my computer taking a massive data dump (pardon the terminology). Aside from those five hours intervals (at the top of the hour) he said my traffic is normal. He said he couldn't determine on his end which program.
Here is the thread containing my logs and the collaboration with racenutalways in looking for any malware. He has pronounced my computer clean.