I am betting that your DSL or cable is really only 1.5Mbps. Most cable/DSL routers only have a 10Mbps port on them because you will never use that much. If you are running peer to peer or even through a router at 100/full then you should be able to peak at 100 send and recieve at the same time. I am guessing if you have 2 PCs hooked up to the same modem then you are using a router or switch. By default Windows is autodetect for speed/duplex. Verify that your router/hub/switch is 100/full. If it isn't then you will not be able to run 100/full.
most dsl and cable router and modem manufacturers use the cheaper 10 meg networking chips and not 100meg as it would be a waste as the connection out the other side to INTERNET is usually only 1/10th that speed anyway (1.5 or less)