Crtl-C, Break, or Ctrl-Break seem to have little effect.
I'm sorry, the documentation isn't entirely clear on that. Ctrl-C and Break only work for the commandline version, not the windows version. I will change the documentation to make this a bit more clear.
I have to give jkdefrag admin priveleges for it to run properly on the OS.
That is true for all Windows versions, not just Vista. Thanks for bringing this to my attention. I will add a bit of text about this to the documentation.