Don't worry, your problem should be fixable.
The installer does not need to be run as admin as it automatically elevates when required. Check if you have the genuine installer. Right click it and open Properties and verify that it has a Digital Signatures tab with the name of the signer as "Ivaylo Beltchev".
It uses Windows Installer (MSI) technology. So another thing you can try is run the Program Install and Uninstall Troubleshooter:
http://aka.ms/Program_Install_and_UninstallYou can run the installer with this switch: /L* log.txt so it will generate a log file. e.g.
"C:\Users\your user name\Downloads\ClassicShellSetup_4_3_0.exe" /L* CSLog.txt