Best way to monitor traffic on my LAN

In my house, I have a combination cable modem/wireless router. I have one computer connected to it directly and 4 connected wirelessly. Generally, this set up works great. However, there are times when the latency on all computers goes through the roof. I know this is expected when any of them is doing a lot of up or down loading. However, it seems to happen frequently when there isnt any major data transfer happening.

I am hoping that someone can suggest a solution for me. I would like a way to monitor traffic on the LAN to determine which, if any, of the computers is causing this problem or if the problem is outside the router. Obviously I would prefer a solution that does not involve acquiring more hardware and that affects as few of the computers on the network as possible.

Thank you very much for your consideration.

 

For MS based network:
http://support.microsoft.com/kb/148942

For a Linux Solution any modern distro will come with the tools necessary to fulfill your request.

 

AAAH! Arthas is in my network!!!

Thanks! I forgot to mention I was running Windows. I will give Network Monitor a try.

 

All your network are belong to us.

lol , cheers mate.