Slow TCP using Windows 2003

Hi,

I'm having a local network that is connected to the internet using a
VDSL modem. I have 2 machines set up. One Windows 2003 Server and one
Windows XP. If I use the same network card in XP I get about 14.3/5.4
MBit/s, but when I use 2k3 I only get around 30% of the download speed,
4.3/5.6. The UDP connection is around 20/11 and same on both 2k3 and XP.
(The upload speed is currently broken as well but at least both 2k3 and
XP give the same result.)

I have tried with a hardware router, a software router (the built in 2k3
router). I tried changing NIC and cables. I've tried turning of NAT and
firewall and tons of other things.

Is there some known setting in Windows 2k3 that makes it slower than XP?
Maybe some service that limits the bandwidth? I've searched high and low
for a solution to this problem without any success. Any and all help is
really appreciated.

erik