The simple solution to this failure of the installer is to run your systems disk cleanup software, which came with the operating system. In this particular case, the OSses designated Temp file directory needed cleaning as it will most likely be chock-a-block aka filled with thousands of files.

Notably, do not go over the top and start removing installer files with the BOINC.msi name or the directories they're stored in such as

c:\Windows\Downloaded Installations\{1098BC5F-7294-4949-811D-443820213767}

The next BOINC installer will be needing them for upgrading, downgrading or uninstalling!