I recommend uninstalling XAMPP and make sure after you uninstall it, you delete the xampp folder, because it usually won't remove it.
Then go to http://localhost/
again and see if you still get the "It Works" message. If so, search for apache.exe and find where the other instance of apache is located, then maybe try running the ApacheMonitor.exe and stopping it, then try deleting that folder. Also, look in your Add/Remove Programs in the control panel and see if Apache is in there. Use that to uninstall it instead if it is.
If you can't delete it, click on your start menu and select "Run" and type msconfig, then press enter. Click on the "Services" tab and the "Startup" tab and see if there's anything in either of them that says Apache. If so, uncheck the box and then restart your computer. You should be able to finish deleting it after your system starts back up. Once that's finished, try re-installing XAMPP and everything should work fine after that.