Error 1720 is a generic Windows Installer error message which you get when there is a problem with a custom action script in the package. To troubleshoot this error:
1. Finish the install and look in the application log in the eventviewer. The eventviewer will have a warning with a source of MsiInstaller. This warning will often show you exactly where in the VBscript the error is.
2. Log the MSI installer. You can activate logging with the following command line misexec /i pathtomsi.msi /L*v “pathtologfile.log”. The log file will be created and added to as the install takes place. When the error occurs leave the error dialog on the screen and look at the log file. The end of the log file will contain the exact action that has caused the error. Look at the last MSI action to run without ending. This will be the action that is causing the problem. Often you can find this action in the CustomAction table (use Orca to open it up) and can fix any syntax errors or even hack the action out of the CustomAction and InstallExecuteSequence tables.