>> What should I do now? How can I clean up the old semi install?
Above error states that another installation is in progress and hence Restart your machine to get the above error resolved.
If you believe that you have partially installed SQL Server, then you can uninstall using Windows Installer Cleanup utility from here to uninstall those traces:
http://www.softpedia.com/get/Security/Secure-cleaning/Windows-Installer-CleanUp-Utility.shtml