By default Windows Xp separates 20% of your bandwidth for quality services like antivirus updates and windows automatic updates and other critical updates which you have set in your computer.
If needed you can make the QoS(Quality of service) service in windows, not to allocate any bandwidth.
For doing this go to,
Start > run (or press win + r keys).
Type gpedit.msc and click ok. (Note that gpedit.msc, which is the group policy editor is not available in windows Home, it is available in windows Xp professional).
Under computer configuration, open Administrative templates > Network > Qos Packet scheduler.
Under Qos Packet scheduler open Limit reservable Bandwidth and select enable and set the Bandwidth limit as 0%.
Doing this will increase the LAN traffic. Also note that when the connection is between two computers running windows Xp, both the computers must have the option enabled to have the effect.