Thanks for the observations, I've replied.
I think Vista simply remembers the fact that you had configured the file with name "iReboot.exe" in that path to be run as an administrator and was (incorrectly) applying that option to the new version as well... out of my hands, unfortunately.
Installing it as administrator shouldn't be an issue - you'd have been asked for UAC elevation to admin rights anyway if you hadn't, because the setup needs admin privileges.