By default windows uses an auto detect feature to determine your link speed. This feature can not handle 100Mbps. to change it. Click Start and right click on "My Network Places" and select properties. On the network connections windows, right click on the adapter for your network connection and click properties. Click the configure button on the Local Area Connection Properties window. Click the advanced tab on the new window. Highlight the one on the list that relates to speed. (My one card is Speed & Duplex, where the other is Connection type, i have also seen media type, and i am sure there are others) If you can figure out which one it is, simply click each one, and look at the value box, your speed property will have the value Auto Negotiate. Change this value to one of the following:
Hubs can only handle half duplex speeds (50mbps in / 50mbps out = 100mbps), because they lack the ability to create virtual circuits as they are stupid, so set this to 100 half duplex (i've also seen 100 half mode)
Switches are smarter devices, and can handle full duplex speeds (100mbps in / 100 mbps out = 200 mbps), because they know which port the destination computer is, and create a virtual circuit to it, so set this to 100 full duplex (i've also seen 100 full mode)
Routers, with buit in hub, see hub section, routers with built in switch see switch section.
Please remember to make these changes on every computer on the network, or you will no notice a change. A transfer is as fast as the slowest connection, in most cases, windows auto negotiate.