As an update to this, I recently installed Vista Ultimate, got the error that POL can't run under the current operating system.
At this point, I copied the entire POL folder from my girlfriend's XP machine to the Vista machine, and overwrote all files. Got a bunch of prompts from the UCA during the overwrite.
Went to launch POL, and still getting the same error.
I disabled UCA, copied the files again (this time to a separate folder so I have a source for XP files) then overwrote the POL folder again. No dice still.
Thinking that having the UCA active during install may be the culprit, so I will uninstall and reinstall with the UCA disabled, and try these steps again. I'll post back if I can get anything actually working.