slow network on special tcp connection

Hi!

I have a special problem. I use a small self developed module that
transfers data to a windows based server. In the developement phase a
normal notebook running WinXP was used for running the server software.
This software triggers the module at least every 100ms and it was
necessary to throttle the software to keep a time frame of minimum 60ms
free on the network between two packages.

Now, while going productive, the software is installed on a windows2003
server machine, based on a modern dual processor system. And now the
trouble starts:
The server isn't able to send packages faster then every 250ms. Second
we are using TCP protocol and even with that, we can see, that the
software tells, that a packet is sent, but the module doesnt receive it.
So something ate the packet between our software and the network interface.

Is there any information related to the network performance or similar
problems with windows2003 server, that could help me. Did I miss an
important service pack or patch?

And, as the server software was developed in Visual C++, is there anyone
knowing about issues with programs the work under XP but have
performance problems in Win2003?

Thanks in advance for any hint or link

Ulrich