Network Problems

Hello,

I have 15 PC on my LAN netwok. And we have internet connection to our network. I'm using linksys router WRT54G2 and DSL modem to connect internet. The speed is 2Mb/s
Some computers have XP and some of them using Vistaand Vista 64 bit operating sytems.
Some computers using static IP on network, its for only purpose to avoid internet access, but still they can access the Lan. And other computer who can access the Internet using dynamic IPs.
And the problem is some times computers can't view each other on network. And it make fileshareing to difficult.
How can we solve this problem?
I want to monitor the Internet traffic on my network, any ideas will be approciated ....
Thanks