Microsoft reserves 20% of your available bandwidth for their own purposes like Windows Updates and interrogating your PC etc

To get it back:

Click Start then Run and type "gpedit.msc" without quotes.This opens the group policy editor. Then go to:
Local Computer Policy then Computer Configuration then Administrative Templates then Network then QOS Packet Scheduler and then to Limit Reservable Bandwidth. Double click on Limit Reservable bandwidth. It will say it is not configured, but the truth is under the 'Explain' tab i.e."By default, the Packet Scheduler limits the system to 20 percent of the bandwidth of a connection, but you can use this setting to override the default."
So the trick is to ENABLE reservable bandwidth, then set it to ZERO. This will allow the system to reserve nothing, rather than the default 20%.It works on Win 2000 as well.