I recently had to do a restore after a model crashed. I remember that I was on BOINC version 5.8.8 when I did the backup, but later upgraded to 5.8.11. Then I did the restore without thinking about the BOINC version.

Now 5.8.15 is available, but I cannot uninstall the current version, nor can I install the new version (which performs an implicit uninstall). Reason: the installer for 5.8.8 is no longer available (somewhere at C:\\Windows\\Downloaded Installations\\...).

Just if someone else gets into the same situation, here is how to get out of it.

Download and install the Windows Installer CleanUp Utility. Run it from the Start Programs menu, then search for the BOINC entry. Select it and click on [Remove], then [Exit]. You should now be able to install BOINC again.

Or you could have installed 5.8.8 over the current incarnation, then install 5.8.15 over that. Usually that works, too.

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I tried that; it refused. Only after running the Windows Installer Cleanup tool I could install 5.8.8.

I think in future backups I will also backup the C:\\Windows\\Downloaded Installations folder.