Poor transfer network rates with 100 Mb Full duplex Local Network

Hello

I get very poor transfer rates when moving large files from computers on my
local network.

The computers are all running Windows XP Home Edition , and 100 Mb Full
duplex LAN card equiped, my router / switch is 100 Mb Full duplex too.

When i try to copy 500 Mb file from one computer to a other, i need about 20
minutes to get the transfer completed !

I noticed Network properties shows only " Network Use = 6% "

How should gain more average, why Network use is not about 100% when
transfering files ?

No more network activities are done when i try to copy files ...

Your help will be much appreciated,
Thanks