The message appears immediately after double-clicking on the Boinc installation file.
I removed everything related to boinc and installing the latest version. Tried to install on different drives: drive c and drive D. Tried to install in different folders. Nothing helps.

Which version of BOINC?
Which operating system?
What is the 'installer' file called?
Where did you download it from?

Version 7.20.2
Windows 10
Downloaded from https://boinc.berkeley.edu/download_all.php

If you try to install the previous version 7.14.2, other errors appear, Error 1706. No valid source could be found for product Boinc.

If you go to Windows in the control panel, programs and components, it displays supposedly installed boinc, but when you try to remove it, a message appears:

The file 'C:\Windows\Downloaded Installations\{C974B9D6-7DB1-4E78-9690-10EB392649A5}\BOINC.msi' is not a valid installation package for the product BOINC. Try to find the installation package 'BOINC.msi' in a folder from which you can install BOINC.

Searching all folders in all drives doesn't turn up anything named Boinc.

Did you have a previous version installed? If so, I would go to the registry and delete all lines containing BOINC irrespective of case and then try again. I have only installed the Windows version under WINE on Linux so unless sure about registry editing wait till someone who knows more about Windows posts before going ahead.

Ah - that helps. We know that one.

The problem is that installing BOINC is a two-stage process. First, it removes (or tries to remove) the previously installed version: only then does it even attempt to install the new one.

Yours is failing at the 'remove' stage. There's almost certainly nothing wrong with the new version you've downloaded.

The easiest way to fix this is to try re-installing the exact same version. If you've still got the installer file you used last time, that's great. If not, you'll need to research the version details - look at your account page at one of the projects you run - and find the download in https://boinc.berkeley.edu/dl/?C=M;O=D.

When you locate the right version, and run it, it should offer to 'repair' the old installation. Let it do that, and run right through to completion. THEN, try installing the new version. The 'remove' stage should work properly after the repair, and continue on to install the new version.

If you can't make it work, come back here and we'll try something else.

I've got to go out for a short while, but - in case you need it - I've found where Jord has hidden the sledgehammer.

BOINC FAQ for Windows installer Error 1325

I tried to talk you through solution 1: there are six more to try if you're still stuck.

Thanks for the help, it was an uphill battle but problem solved. I installed boinc version 5.10.4 and after that I was able to install the latest version. Rosetta runs on an old 3930k.

I installed boinc version 5.10.4 and after that I was able to install the latest version. Rosetta runs on an old 3930k.

My FAQs are a little outdated, but next time you (or anyone reading this with the same problem) runs into this problem, use the Microsoft Troubleshooter to remove the program that cannot be uninstalled (choose BOINC), then install the new version.