From: brikeh on

Some of us may find that after installing Windows XP, the Internet
connection gets slower. A possible reason for this is the QoS (Quality
of Service) installed. This service reserves 20 per cent of the
bandwidth for itself, even with QoS disabled. In order to remove this
reserved quota, you will have to make sure you are logged on as
Administrator. Go to Start > Run and type gpedit.msc. Expand the local
computer policy branch, the administrative templates branch, and the
network branch. Highlight the QoS Packet Scheduler in the left window
and in the right window double-click the limit reservable bandwidth
setting. Then, on the Setting tab, check the enabled item and change the
Bandwidth limit % to read 0. After doing this, you should immediately
notice a boost in your Internet connection speed.

You also need to verify that QOS packet scheduler is enabled in your
network connection properties done by going to control panel > network
connections > right click on your connection > properties > check mark
for QOS.