Ran into this tweak for XP the other day. In case it has not been posted before thought I would throw it out there.

Dramatically speed up Windows XP networking (Professional Edition only) NEW!
For some reason, both the Windows XP Professional ships with a new networking service called Quality of Service (QoS) enabled by default, even though this service is only of use in large corporate networks. At home, and in smaller offices, QoS is best left disabled, however, and doing so will speed up networking operations up to 20 percent. Confusingly, turning off QoS in Networking Properties will not actually free up the lost bandwidth. Instead, you must load up the Local Group Policy Editor (see the tip Use the ultimate configuration tool, below, for more information) and tweak some settings to get QoS out of your system for good.

Here's how you do it. Open the Start Menu and choose Run, then type gpedit.msc in the text box and hit ENTER. This will load the Local Group Policy Editor (LGPE). Once the LGPE is loaded, expand the following nodes in the left tree view:

Computer Configuration | Administrative Templates | Network

Then, select the QoS Packet Scheduler entry and double-click the setting titled limit reservable bandwidth, found on the right side of the window. On the Setting tab of the Limit reservable bandwidth Properties dialog, select the Enabled option. Then enter 0 in the Bandwidth limit (%) combo-box. Click OK, then exit the LGPE. No reboot is required to enable this change.

Note: If you look at the properties dialog for your network connection(s), you should still see the QoS Packet Scheduler listed on the General tab. If this item is not present, QoS is still taking up 20 percent of your bandwith! You need the packet scheduler installed to ensure that no bandwidth is allocated.