I wrote this PowerShell function to change drive letters with PowerShell and DiskPart. Other methods posted online utilize the Win32_Volume WMI class which is not available in Windows XP. Using DiskPart makes the function compatible with Windows XP/2003 as well as Vista/2008/7.
You may also be interested in my PowerShell Script to Build a Custom WinPE v3.0 ISO posting. It makes what can be a difficult process much easier!