One thing that makes Windows such an effective platform is its great flexibility: Nearly everything that (on the surface) seems to be hard-coded can be tweaked, turned off, or reconfigured. To that end, Windows XP has the Group Policy Editor, a tool that lets you not only tinker with the operating system but also lock down many of its vulnerabilities.
The GPE has been around since Windows 95, but because its a power tool, Microsoft has always chosen to hide it (much like the Registry Editor). This is understandable, as there is some potential to wreak havoc, but the GPEs point-and-click design makes it much safer than changing the same settings in Regedit.
Note that the settings discussed here are available in Windows XP Professional; some may not be present in XP Home.