If waiting, as Teggit suggested, doesn't help, open the task manager (CTRL+ALT+DEL or otherwise, e.g. by right clicking on your task bar) and make sure that the installer only appears ONCE in the list of running processes. If it appears twice or more, end all those installer tasks, and move the file somewhere else (you should be able to move the installer to a different location, right? After all, it's "just" a file), then start it again.