Your NIC will default to "Auto Sensing" the speed of the connection. So will your switch. All too often your system will not perform as well as it should and if you are new to networking you may think your system is just about the fastest thing around, until you see how fast it can really be.

Your NIC can run in one of two modes, half duplex or full duplex. Half duplex is much like a two-way radio, where only one person can talk at a time. If two computers talk at the same time you get a "collision" and one of them must try to talk again before the other does. It's not terribly efficient but when switches were extremely expensive it was the only way to go.

A full duplex connection can transmit and receive at the same time over different paths, thus eliminating collisions and running effectively at twice the speed, or 200 MB. The trick, then, is to ensure that the NIC is running at 100 MB full duplex.