By default (even with QoS disabled) Windows XP and above reserves up to 20 percent of your connections bandwidth. To override this reserve take the following steps.
* Start > Run > type gpedit.msc
* You will see [Local Computer Policy]
* Expand the [Administrative Templates] branch
* Expand the [Network] branch
* Highlight [QoS Packet Scheduler]
* Double-click [Limit Reservable Bandwidth]
* Check [Enabled]
* Change [Bandwidth limit %] to 0 %
* Click [Apply] [OK]
Make sure you Log on as Administrator, not as a user with Administrator privileges.