I'm going to assume you do not have Managed Network Devices. Meaning you can't login to and monitor your switches.
So, how about some low-tech ways to figure out the problem? Tell your Mate, your Mother or your Cat you will be late for dinner.
Tell all employees to leave their computers running for the night. After everyone has gone home, look at the lights on the switches and any routers on your network. Identify which ports on the switches are still active, then figure which workstations they connect to. Now you know which PC's are generating all the traffic. IF all the activity lights are flashing at the same time, you have a broadcast problem. Running tcpdump all by itself on the command line will show these broadcasts and their source MAC. If you have multiple switches connected together, make sure you have not created a "loop" in the wiring. You're only talking about 25 hosts, all this should not take long.
Then disconnect everyone from the Internet except yourself and see if your network speed improves. If you believe it is still too slow, you will probably need to talk to your ISP. Depending on what router/gateway device you are using to connect to the Internet, there are monitoring tools like mrtg that will monitor how much of your bandwidth you are using. You may be paying for a T1 1.54MBPS frame relay link, but how do you know if you're actually getting it? Are you hosting a public web server at your location? See if your ISP does server or rack hosting to move the server off your connection to the ISP. Usually much cheaper than paying for more bandwidth.
Last edited by Suncoast; 07-24-2009 at 05:01 PM.