I had an application that when it would install kept giving me an ISScript.msi error I tried loading several versions of the ISScript and issue persisted so I found this little tool

I know the tool is meant for office but it does fix ISScript errors for other apps after you install the tool.  Launch the tool find the application giving you the error in the list click on the app and then click remove.  Then reinstall the application and the issue should be resolved.