Why do you need to change the speed of your network card? Most network cards should auto-negotiate to the right speed if they are capable to handle that speed.
If you really have to force it's speed then you should look into changing the duplex mode. It's not always the same for each network driver so I suggest you look at the following site and find out how to do it for the network driver which you are using.
http://www.scyld.com/network/index.html
network card speeds
