Microsoft Knowledge Base Article 825668 contains the following summary:

This article discusses Plug and Play (PNP) enumeration in Windows Server 2003 and Windows XP. The article contains information about how Windows Server 2003 and Windows XP assign drive letters to hard disk drives when you run Windows Setup on a computer with multiple hard disks. This article also discusses how PNP enumeration behavior in Windows Server 2003 and Windows XP is different from PNP enumeration behavior in Windows 2000.