If you are using Windows, go to "Computer" (or "My Computer" pre-Windows-7), then click "Uninstall or change a program" (or a similarly named button) at the top, just under the button and address bar. That will give you a list of all programs you have. Find EU3 and try removing it. If that does not work, you can also try removing the relevant entries from the registry, BUT if I remember correctly, Paradox games don't usually require that. Which means that you should be able to install EU3 afresh despite not having uninstalled the previous one cleanly. Just choose a different folder for the installation so that there is no cross-writing of files.