Windows XP, just like Win2k & NT before it, runs many background Services for a wide variety of tasks, e.g. Plug & Play support, Print Spooler & even Task Manager.
There are ways of optimizing the usage of these Services and configure your machine for improved performance and even security by modifying their respective Startup types, and in some cases disabling them altogether. 3D Spotlight"s tweak guide will take you through a multitude of different Services, explaining how and what they work for.