for an uninstall, your command will look something like:

Shell 'C:\WINDOWS\uninst.exe -a -y -f"C:\Program Files\Pivotal\Relation\DeIsL1.isu" '

The -a option might help... or might not.

http://support.installshield.com/kb/view.asp?articleid=q100021

But ultimately it is based on the version of the installshield app that packaed it.

So.. SOME APPS DO NOT HAVE A SILENT UNINSTALL and there is little that can be done short of a brute force removal, del the reg keys, the folder , the shortcuts... not all apps are created equal
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