hmm... well, all i can say is, if it runs under administrator just fine, then you've got a local configuration problem that's preventing non-privledged users from being able to run it. This wouldn't be a Spark issue of course, but rather a Windows Domain issue (GPO, whatever...), or if you're not in a domain, then local user permissions.
To run, Spark must create a directory under %APPDATA% for the user running Spark. This stores configurations, transcript history, and a bunch of other stuff necessary for Spark to run correctly. Perhaps your un-privledged users are locked down and unable to create new directories in %APPDATA% ?