I could give you some idea but it may not help.
Check the network cable, make sure it's not bend, cut or damage and make sure it's not over 100m (I doubt it's over that length)
Switch the network card.
Use another switch port on the router
Run a spyware scan/check
Run a packet analyzer (ethereal) on the slow machine to capture and analyze the connection when the computer is idling.
Check for/configure any firewall/Anti-virus application running.
Run Linux for servers, not 98.