Joomla Uninstall

I have a problem here and I just wanted to start over from scratch. I had originally installed Joomla, but wanted to try Drupal. Well, I couldn't figure out how to uninstall Joomla, so I just deleted everything using an ftp client. Now when I try to install Drupal, it says "Installation failed. You cannot install the application because there is another application installed in the root folder of your site."

How do I fix this?

You can find the link to unistall an application in you website control panel by clicking the app name in the dashboard.

I tried that. I didn't work. I finally figured it out. I had to download Joomla to my computer and upload the files with an FTP client. Then the uninstall link worked.