Don't know if anyones aware of this. xp has a out of the box 20% resesrvable bandwidth cap. i stumbled across this tweak yesterday, simply sets your cap at 0%, enjoy

If you have XP pro you can try tweaking your QoS reservable bandwidth settings using GPEDIT.MSC
Go to START-RUN and enter GPEDIT.MSC then go to Local Computer Policy -> Computer Configuraton -> Administrative Templates -> Network -> QoS Packet Scheduler. Look in the right side and select Limit reservable bandwidth. Double click on Limit reservable bandwidth. and set it to 0%. reboot.