Because I don't have a vista (or 7) machine to test it on. It's fine if you're right, and I don't mind being wrong. But that's just retarded if the program can't even run (it's just an IM program for cryin' out loud!) without running under an admin account.
Oh well, you're an admin so you can run it.
I guess you could always right-click and run as admin if you're ever logged in as a standard user.