So if your partition is already setup then it should be easy due to negating the need for diskpart.
so just
ex.
mkdir %driveletter%\minint
Xcopy thefiles %driveletter%\minint
Xcopy ntdetect %driveletter%
easy man just try it out.
QUOTE
To create a bootable WinPE disk (32-bit versions of WinPE)
***Please note this is only the instructions on how to install WinPE and WinPE only to the hard drive, you must modify these instructions to get a "Dual boot"***
Boot the destination computer into WinPE using a WinPE CD.
Create a formatted and active partition.
On the active partition, create a directory called C:\Minint. You must name the directory "Minint".
Copy the contents of build_location\i386 to C:\Minint.
For example:
xcopy "C:\Build.x86\i386\*.*" C:\Minint /S
where C:\Build.x86 is build_location.
Copy Ntdetect.com from build_location to the root of the destination hard disk.
For example:
xcopy "C:\Build.x86\ntdetect.com" C:\
where C:\Build.X86 is build_location.
On the destination hard disk, copy C:\Minint\setupldr.bin to C:\ntldr.
For example:
xcopy "C:\Minint\setupldr.bin" C:\ntldr
Restart the destination computer. The computer starts using WinPE.