Go into Device manager
Expand the Network Adapters ,click the + mark in front of the line.

R-click the nic and select properties.
Select the Advanced tab.
Scroll down and find Speed and Duplex.
Single click the line and in the drop down set it for 100Mbps Full duplex.

Full duplex is like a telephone, you can talk both dirctions at once. Half duplex is only one way at a time.

Not the half duplex shown here.



For XP, this might be easier.
http://www2.slac.stanford.edu/comp/w.../nic_speed.htm