Despite having found some things about this, I can't make it work (it is fine on all *nix systems I use). I would like to have something like this in my org-mode file, and then see the diagram below it.
#+begin_src plantuml :file test.png :exports results
The root problem is clearly that java on windows doesn't like windows paths.
I won't go into detail on the unsuccessful things I tried, but they include changing various variables, modifying the code that creates the java command to execute, and writing a wrapper for java.
Please take pity on me and give me an idiot-proof cookbook solution - I'm sure a simple one exists.