You've another instance of Tomcat already running. You can confirm this by going to http://localhost:8080 in your web browser and check if you get the Tomcat default home page or a Tomcat-specific 404 error page. Both are equally valid evidence that Tomcat runs fine; if it didn't, then you would have gotten a browser specific HTTP connection timeout error message.
You need to shutdown it. Go to /bin subfolder of the Tomcat installation folder and execute the shutdown.bat (Windows) or shutdown.sh (Unix) script. If in vain, close Eclipse and then open the task manager and kill all java and/or javaw processes.
Or if you actually installed it as a Windows service for some reason, open the services manager (Start > Run > services.msc) and stop the Tomcat service. If necessary, uninstall the Windows service altogether. For development, just the ZIP file is sufficient.