By default, most systems have some bandwidth reserved for nothing. You can change this the following way: (Note: this only works in Windows XP Professional,windows 7)
- Open gpedit.msc from the run box in the start menu.
- Navigate to: Local Computer Policy > Computer configuration > Administrative templates > Network > QoS Packet Scheduler
- Open Limit Reservable Bandwidth
- Check the enabled box and change the Bandwidth Limit to 0%.
Depending on your system, this might need a restart to start working.