User: achille.cristallini5@tin.it (achille)
I have a system with Windows 7-64 Home
Boinc version installed: boinc_7.14.2_windows_x86_64
I tried to install the version: boinc_7.16.5_windows_x86_64
During the installation the message is given: the file boinc.msi cannot be found.
I completely uninstalled the boinc_7.14.2_windows_x86_64 version and removed any folder, file or reference to boinc from the C: \ disk and the registry.
Attempting to install the new version gives the message: impossible to remove an older version.
What to do?

Use https://support.microsoft.com/en-us/help/17588/windows-fix-problems-that-block-programs-being-installed-or-removed to get rid of the old uninstaller file.
Alternatively, reinstall https://boinc.berkeley.edu/dl/boinc_7.14.2_windows_x86_64.exe to fix the uninstaller file, then uninstall it again.

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No result with Windows fix problems
Impossible to install Boinc 7.14.2 Error message: impossible to remove an older version

You must've deleted something unwanted when you deleted parts of the registry. Have you tried going back to a previous restore point, from before you uninstalled BOINC?
You can also try third party apps, such as Revo uninstaller free, or a favorite of mine HiBit Uninstaller.

And if that doesn't work, you're probably looking at fixing Windows.
Try first to do SFC /scannow from an elevated command prompt. You may need your Windows CD/DVD handy for that.

In the future, don't just dive into the registry and go delete things left and right without making a backup of that registry or key.
We have had this problem with Windows losing sight of where it keeps the MSI files for ages, we also know the solutions (See W). Better come ask first before you destroy your system.