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.
Then go to:
Local Computer Policy
Then Computer Configuration
Then Administrative Templates
Then NetworkThen 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. 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%.

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Note:This trick works only for windows xp sp2 professional and windows 2000 professional.It does not work on windows xp sp2 home edition.