Internet traffic monitor utility

My pc seems to download 40+ gigs per month by itself, while idling for 4 hours my pc downloaded 362 megs of freeking packets from what seems to be no freeking where :S
I have been thinking my isp was riping me off and just trying to scam me, but I recently switched the modem to my other pc with XP installed, whereas my other pc has Vista Home Prem. and in 9 hours and 43 minutes, i only downloaded 74kb of packets.

Is there a program I can get, preferably freeware, that monitors ALL internet activity, tells me what ports and downloading and from where, and uploading plus from where, plus many other things I would need for a situation like this one?
I need the most diverse program possible for this, regardless of how hard it would be for a noob with no experience in this what so ever is.