What speed do you want to run it at, and why do you want to slow it down ? If you have OS10.3.3 or above you can set it to 10 Half Duplex, but that is as slow as it goes. If you are connected to a 100 Full Duplex Connection and the speed is set to Auto Macs detect it as 100 Half Duplex and run very slowly.
Open a Terminal and issue the ifconfig command and that will tell you your current connection speed