Client/server

I have a client set up on a standalone server by himself. NT4.0 server, 2000 client. Both have 100Mbps NICs, I download from the server at 100Mbps but when I upload it slows down tremdously. What could be the problem. I have both cards monitored andthe connection both say 100Mbps also they are connected to a 100 Mbps hub.