I would advise you to run some additional tools other than your virus scanner:
- RUbotted, free from Trendmicro.com (install and keep running on all PC's, it will pick up very quickly any outgoing stuff that a virus scanner will sometimes NOT pickup. This might find something is running that is sending out information you are unaware of.
- Hijackthis (google it). Run it and look for any weird DLL's that are hapening at startup - check the DLL's/ I had a malware one in a startp DLL once that was never picked up by AVG or spybot
- A free rootkit checker (sophos do one I think) run it to check for stuff that, again, some virus scanners don't find
- Spybot, search and destroy (again free). Run at least once a week,
Also, if you are using norton or Mcafee, you may want to consider changing virus scanner.
In addition to known filesharing programs there are other downloaders that you may not realise are P2P. For example some MMORPG loaders (world of warcraft, lottro), video download service running kontiki which never shut down (sky, bbc etc video downloaders). All of these will have propagated your IP and someone other downloader might be running while you are asleep with cached IPs.
There is a lot more to security than your router
