Hi,
you might start by backing up your xbox eeprom. With that eeprom.bin file, you can experiment as much as you want without the need to do a hotswap.
xboxhdm takes the stuff that was in it's C and E folder at the moment you created the iso and copies that to your xbox C and E partition when it prepares the disk.
If you run the ndure script before you create the iso, it will alter the C and E folder contents so that your xbox starts softmodded with a alternative dashboard.
So, basically, you should start with fresh M$dash files (5960 if I remember well) in your xboxhdm C and E folder. You should run the Ndure script on them. Make sure you select the right options in Ndure. (Not the basic setup)
You can get the M$dash files if you extract a Slayers Iso. I use the Qwix program to extract Xiso's.
Ftp your eeprom backup from your xbox to your pc, and place it in the xboxhdm eeprom folder and the e:\backup folder. Make sure it is named eeprom.bin without capital letters.
If everything works out fine, you should end with a softmodded xbox. If your c drive is empty, it is your shadowc. Just format it and copy the c M$dash files over to it.
Changing the dashboard to xbmc is something you do afterwards. You ftp the xbmc package over to the xbox. It comes with a sort of shortcut. You can replace the e:\dash\default.xbe by that shortcut and the package should run instead of unleashx.
regards.