Aha, I've cracked it. I previously redirected the command prompt to my 
most commonly used path via a registry entry called Autorun. Since 
that's worked on every other computer I've used without a hitch I didn't 
even think of it until I noticed Ocra's pathing code. That must have 
been affecting the build process because now that I've disabled that key 
value, everything works fine.
I guess the moral is: Playing with the registry may have unforeseen 
consequences!

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