Please advice. How can I determine which computer in our office has the most bandwidth. This is the scenario. We have 25 PC and there are a times our internet is slow. As I checked during a single PC the DSL provider has no problem. I assume that some of them is downloading a movie which is wrong because its affect in our network. Please advice, what software or application can solve my problem?
The firewall/router can probably be configured to meter traffic for individual IPs from the local network. Maybe this points you in the right direction: http://www.catonmat.net/blog/traffic-accounting-with-iptables/
The second on looks promising actually. You could then change the gateway for all office PC's to the one running the monitoring program which in turn forwards the traffic to the Internet. This would be ok for a temporary solution at least.
First, you'd better check whether your router supports "bandwidth monitoring". If you're a technical guy, you can flash your router's firmware into openwrt to get this kind of features.
If you prefer software solution, there is a lot programs you can try:
WFilter Free -- freeware for network monitoring PRTG -- free version available for monitoring