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Possible quick fix:
I would try re-running the installer, rebooting if prompted, then try the uninstaller again.
That may be enough to correct things and make the uninstall work.
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If you still have problems:
The steps in Problems installing Opus for repairing the similar InstallShield error while installing (clearing the InstallShield and temp folders) are worth a try.
As with the "quick fix", you would need to re-run the installer afterwards, then the uninstaller.
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Manual uninstall:
If Opus was set to replace Explorer and the uninstaller is failing, you can manually turn off Explorer Replacement via a .reg file.