Now, that's strange, isn't it?
My thought would be that the service wasn't fully uninstalled. The good news is that since you haven't configured anything, you'll only need to wait through the recompile time.
But what I would do is to take the "hammer" approach to uninstalling:
* Stop the service (I think it's "/etc/rc.d/ispconfig_server stop" in FC)
* Run the /root/ispconfig uninstall script again
* and for good measure, I'd look in /root and in /home (or where ever you put the user/hosts data) and would manually remove any ISPConfig-related subdirectories.
* To thoroughly make sure you don't have a leftover ISPConfig running, reboot the machine. Yes, it's unneeded, but then you know you have a clean box to reinstall on.