monitoring network

hi all, i'm pretty new here but was wondering if i could get some help with a network question. i'm running a home network with one mac and a pc connected to the same router. are there any freeware utilities or terminal scripts/commands that i can use to monitor connections to and from each computer on the network and the internet? i know how just to monitor the connections made on my mac, but its the other pc that im wondering about how to monitor. i suppose i can just do it directly from the pc, but it would be so much more convenient if i can do it without leaving my pb.

i want to know since being in a college town there are about 10 nearby wireless networks i can detect and i dont want anyone leeching into my network (although it is password protected and broadcast is turned off). yes i guess im a little paranoid, but i figure its good to be sure.

thanks!