Windows XP Home/Professional is a client operating system with no heavy-duty server-specific tasks in mind. Database management system is clearly one of the daunting tasks. Therefore, not allowing SQL Server 2000 to be installed on Windows XP is rational. You can install it on Windows 2000 Server up or Windows 2003 Standard up.

Alternatively, you can modify some registry settings to make Windows appear as server. However, I do not know which key to change and this is not advised.