Thanks all for your suggestions,
Now i make the point of the situation,
My first problem was with a machine with Win XP Pro and the transfer rate with the SME server, and i think to have tried all possible software and hardware tweaks.
After 2 days of tests: new machine and new installation.
Pentium 4, 512MB Ram, 80GB Disk, Realtek 8139, Nvidia GeForce MX, Windows XP Pro installed from original CD with service pack 1.
I have connected Windows to the network with dhcp, no access to domain, only local user, no change to the windows config: same problem!
Downloaded and installed all the updates from microsoft, tested all the tweaks.
Nothing
New test: Pulled out from dust the old server, SME server 5.5 without updates!
Connected to the network and tested the transfer: same result.
Ok Sirio, maintain the calm!
Latest test: Other machine with Windows XP home, i dont remember what hardware, but network card RTL 8139: it goes fast like a rocket in both senses.
I am thinking to kill to me!
Just a hint: Windows XP is in italian(both Professional and Home)!
If i find an english version, i want to try!
Someone thinks that installing SME version 6 it can change something?