Pump Up the Bandwidth!

Windows XP Professional has a feature built into it called QoS
(Quality of Service). This feature allows the operating system
to optimize bandwidth allocation for different services.
While books are filled with incomprehensible information on this subject,
what you need to know is that this feature actually throttles back your
available bandwidth. QoS reserves 20% of the available bandwidth on any
connection for itself. This means you can't use it for downloads
and LAN connections! You can cut the QoS brick tied to the back of your
network connection by performing the following steps:

Click Start and then click the Run command.
In the Run dialog box, type gpedit.msc in the Open text box.
Click OK.
In the Group Policy window,
expand Computer Configuration,
and then expand Administrative Templates.
Expand Network and click on QoS Packet Scheduler.
Double click on the Limit reservable bandwidth entry in the right pane.
Select the Enabled option and then type 0 in the Bandwidth limit (%) text box.
Click Apply and then click OK.
Close the Group Policy window and restart the computer
(not sure if you need to restart the computer, but do it anyhow).
QoS is enabled by default on all adapters. If you disabled it for some reason,
make sure it is enabled, or else it's possible that the 20% limitation
will be enforced anyhow. There's not too much documentation of this feature,
so if you find it improves your bandwidth, let us know!