The installer fails looking for the MSI file at "C:\Windows\Downloaded Installations\BOINC\8.2.4\f0a7c588eb7f77fbfadfa8308ce7cfba\BOINC.msi" with the error message "not a valid installation package for the product BOINC". I was running MilkyWay@Home at the time. I have tried downloading again in case there was a corruption. I have copied the msi file to another directory in case it was windows trying to protect the installation directory noted above. Here is the entry from Event Viewer,

- <Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
- <System>
<Provider Name="MsiInstaller" />
<EventID Qualifiers="0">11706</EventID>
<Version>0</Version>
<Level>2</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2025-08-18T11:30:34.9952674Z" />
<EventRecordID>373158</EventRecordID>
<Correlation />
<Execution ProcessID="0" ThreadID="0" />
<Channel>Application</Channel>
<Computer>E2180</Computer>
<Security UserID="S-1-5-21-3609417406-1899880879-4180427455-1011" />
</System>
- <EventData>
<Data>Product: BOINC -- Error 1706.No valid source could be found for product BOINC. The Windows Installer cannot continue.</Data>
<Data>(NULL)</Data>
<Data>(NULL)</Data>
<Data>(NULL)</Data>
<Data>(NULL)</Data>
<Data>(NULL)</Data>
<Data />
<Binary>7B45453230393631462D463537462D344145312D423239392D3330443239343332333934417D</Binary>
</EventData>
</Event>

My system is:
Windows 10 Pro version 10.0.19045 will all the latest updates
CPU is AMD Ryzen 9 5950X
32GB RAM

Have you tried installing with administrator privilidges?

That would not solve problem and possibly could lead to other problems.

TimAtHome, The proposed solution is to treat it just like the "Windows Installer Error 1706: Setup cannot find the required files"
Link to the wiki solution

Try this and reply with your results.

Good idea @Dave but sadly it made no difference.

Success!

I had to remove all instances of BONIC from the registry, there are a lot. Then check %AppData% and remove all instances of BONIC there. This time it worked and even picked-up my current project work.

An observation, to me it seems excessive that in the registry the ".mo" files have their own separate key.

Thanks for the suggestions @Dr Who Fan and @Dave