Created an account just to help you. If you are using the boot.wim from a windows setup dvd then there are 2 images inside that wim file. The first image winre and is not booted to by default. I just delete it if I'm creating a PE from the setup, then I mount the second (now first) image and delete the setup.exe from the root. Once the image is saved it will boot to the command prompt instead of setup.