Copying files using Winsock 2.0 and the IOCP

I am copying files from a server to clients using the IOCP technology.
I have this proceeding full tested, and it is working well.

I am having some problems, just in some computers.

In some computer, apparently with the same configuration and with same OS,
the copy are very very slowâ?¦ one two minutes for just some kbâ??s (when I copy
the file directly in network between server and those computers the speed is
normal).
In other computers the speed is always normal.
All computers have the same antivirus, and similar programs installed.
What could be slowing the speed in some computers?
Thanks.
Rui