QoS (Quality of Service) is a networking subsystem which is supposed to insure that the network runs properly. The problem with the system is that it eats up 20% of the total bandwidth of any networking service on the computer (including your internet connection). If you are running XP Professional, you can disable the bandwidth quota reserved for the system using the Group Policy Editor [gpedit.msc].

You can run the group policy editor from the Run command line. To find the setting, expand "Local Computer Policy" and go to "Administrative Templates" under "Computer Configuration." Then find the "Network" branch and select "QoS Packet Scheduler." In the right hand box, double click on the "Limit Reservable Bandwidth." From within the Settings tab, enable the setting and then go into the "Bandwidth Limit %" and set it to 0%. The reason for this is that if you disable this setting, the computer defaults to 20%. This is true even when you aren't using QoS.