Internet Data sniffing?

[Tarkus]

Lately, I've been kind of curious as to what kind of data various programs have been sending in and out of my computer. Although I see how much data is being sent out by tiny firewall, I'd like to be able to actually manipulate and see what kind of data is going in or out of a particular port.

Are there any libraries either for VB or C that I could use (preferably VB) that would enable me to do this?

[nex]

There's a windows pcap library (http://winpcap.polito.it/) that you could look in to (libpcap has also been ported to win32 I think, I'm not sure if that's a seperate project or what). I'm fairly certain you can use the DLL with whatever language you want, but the developer pack probably only includes C headers and examples, so that'd likely be the most straightforward language to use. But for just looking at the data, I'd recommend Packetyzer (http://www.packetyzer.com/).

Actually modifying the packets as they go through is probably a lot more complicated. I'm not sure if winpcap has any methods for doing it.