Issues/Symptoms:

1. The Revit 2014 / 2015 installer you've downloaded from Autodesk is corrupt and will not install correctly. Download your Autodesk installers from the "Autodesk-Student" share here: Software Downloads
2. You have uninstalled Revit 2014 / 2015 / 2016 and need to reinstall.  When you attempt to reinstall Revit 2014 / 2015 / 2016, the installer reports Revit as already being installed, which you know is not true, since you just uninstalled Revit 2014 / 2015 / 2016.  You're unable to install Revit 2014 / 2015 / 2016 again.
3. Templates and families not installed Revit 2014 / 2015 / 2016?
   1. http://knowledge.autodesk.com/support/revit-products/troubleshooting/caas/sfdcarticles/sfdcarticles/Revit-Templates-and-families-not-installed.html 

Resolution:
For a clean Revit 2014 / 2015 / 2016 / 2017 installation, uninstalled any/all Revit related software from the Windows Control Panel.  Then verify that Revit related registry keys listed below are removed from the system after Revit is uninstalled.  Typically the Windows Installer clears these keys, but not in this case.

1. Click Start

2. In the Start Menu either in the Run Box or Search box type regedit and press Enter.

3. If prompted by User Account Control, click Yes to open the Registry Editor.
4. Once opened successfully you should be in the Windows Registry Editor Window, similar to the example shown below.
   
   
   Tip: If you have restricted access to the Windows computer you're logged into you may not be able to access the Windows Registry.
   
   

5. How to browse the Windows Registry (***IF YOU ALREADY KNOW HOW TO BROWSE THE REGISTRY, SKIP TO STEP 8***)
   

   1. When most users need to edit their Registry they're given the location or path of where the Registry value is contained and what to change. In the example below, is an example path of a commonly accessed Registry subkey. To browse to this location you first start by opening the HKEY_LOCAL_MACHINE key (folder). Once in this key you should see the SOFTWARE folder and within this key contains the remainder keys listed below.
   2. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
   3. Once in the above path you'll have a window similar to the example below. In the example below, you can see four different string value keys, which in this case are pointing to each of the programs Windows runs each time this computer starts.
      
      
   4. 
      
      

6. How to edit a Windows Registry value

   1. To edit a value, double-click the name of the value you want to edit. For example, if we were to double-click the 'IntelliPoint' Name a new window would appear that would allow us to change the value of the Data. In this example, we could change the location of where the file is located for the IntelliPoint driver.
      
      

7. How do I delete a Windows Registry value

   1. To delete a value highlight any Registry Name and then press the delete key on the keyboard. For example, if we did not want the IntelliPoint to load each time Windows starts we could highlight IntelliPoint and then press the delete key.
      
      
8. Delete the following Revit / AutoCAD related registry keys from the Windows Registry:
   

   1. If you are unable to find any of the keys below move on to the next one.

      
      Revit 2019:
      HKEY_CLASSES_ROOT\Installer\Products\0A4B643700910150000007C5D0680240
      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\0A4B643700910150000007C5D0680240
      
      Revit 2018:
      HKEY_CLASSES_ROOT\Installer\Products\0A4B643700710150000007C5D0680240

      Revit 2017:
      HKEY_CLASSES_ROOT\Installer\Products\0A4B643700710150000007C5D0680240
      
      Revit 2016:
      HKEY_CLASSES_ROOT\Installer\Products\0A4B643700510150000007C5D0680240

      AutoCAD 2016:
      HKEY_CLASSES_ROOT\Installer\Products\7D2F3875100F0000102000060BECB6AB

      
      Revit 2015:

      HKEY_CLASSES_ROOT\Installer\Products\0A4B643700410150000007C5D0680240

      HKEY_CLASSES_ROOT\Installer\Products\0A4B643700411150409007C5D0680240

      HKEY_CURRENT_USER\Software\Autodesk\Revit\2015

      HKEY_CURRENT_USER\Software\Autodesk\Revit\Autodesk Revit 2015

      HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\Revit\2015

      HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\Revit\Autodesk Revit 2015

      
      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\0A4B643700410150000007C5D0680240
      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\0A4B643700411150409007C5D0680240
      
      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{7346B4A0-1400-0510-0000-705C0D862004}
      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{7346B4A0-1400-0511-0409-705C0D862004}
      
      

9. Once you've deleted the keys above, close the Windows Registry Editor window, and reboot your computer.
10. Try reinstalling Revit 2014 / 2015 / 2016 / 2017.
11. Additional Resources:
    1. 2017: How to Complete a Clean Install and Uninstall of Revit products