Try putting the book in the same folder as the script and not putting 'python' at the beginning of your command string just mobidedrm drmdbook.prc undrmdbook.prc PID (.prc could also be .mobi or .azw). Also make sure you're in the folder where the script is before starting (for example C:\mobi>).
Mods, if this is TMI feel free to delete or edit.
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