Well that was a very long winded way of simply saying that you cannot uninstall ST8 and are getting an Error 1721 when attempting to uninstall.
At this point you will need to manually clean up the Solid Edge install. From Solution Center article 002-7005960:
Manually uninstalling Solid Edge after failed uninstall
When attempting to uninstall Solid Edge, the uninstall fails and Solid Edge is
not fully uninstalled. How to clean up and manually finish uninstalling Solid
Edge after a failed uninstall?
If you have issues with uninstalling Solid Edge then you will need to perform a
1. Manually delete the Solid Edge program folders typically in C:\Program Files
or C:\Program Files (X86).
2. Manually delete the Solid Edge user preference folder %APPDATA\Unigraphics
3. Manually clean up the registry by deleting any Solid Edge registries in the
4. Reboot the workstation.
Solid Edge should now be completely uninstalled and ready to reinstall Solid
Please note that if you have other Siemens PLM software installed you may not want to broadly delete the "Unigraphics Solutions" hives and instead selectively delete all Solid Edge entries under the various "Unigraphics Solutions" hives.
I've actually seen this before where Solid Edge removes all files but fails to tell windows that it's uninstalled.
You can force this in the windows registry, but first, can you make sure that SE is completely wiped from your machine?
You can do it like this:
After that, go to the windows registry again (Press Windows Key + R, Type ‘%RegEdit%’ and hit OK)...
You'll then need to find which key corresponds to Solid Edge, and delete it...
Select the individual 'folders' until you find the one that says 'Solid Edge ST#' on the right.
Then right click the 'folder' and delete it.
This will de-register Solid Edge from your programs
Hope this helps