File copy WinXP to 2003 Server

I have the folowing problem,

A network with 1 2003 Server, 3 Win XP clients 2 Win 2000
clients.

When copying files, server to Win XP client it goes fast.
When copying files, Win XP client to server is goes realy
S-L-O-W.

You can go out for lunch when you must copy 50 MB.

When copying files, server to Win2000 client it goes fast.
When copying files, Win2000 client to server it goes fast.

When I copy one file of 50 MB it's goes fast both ways on
a Win XP client.

I tried TcpDelAckTicks in the registry, with no luck.

MC