Microsoft reserves 20% of your available bandwidth for their own purposes like Windows Updates and interrogating your PC etc.. You can 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
->Computer Configuration
->Administrative Templates
->Network
->QOS Packet Scheduler
and -> Limit Reservable Bandwidth

Double click on Limit Reservable bandwidth. It will say it is not configured, 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 0.
This will allow the system to reserve nothing, rather than the default 20%.It work with Windows XP sp2 PROFESSIONAL only.