I have found in "C:\windows\Downloaded Installations" many backups on old versions. it should keep only last one.
got win7 64

Yeah well, here's the thing. While it might be technically possible to do this, it's a bit impossible for the Microsoft Installer (that makes those .msi files) to determine which one is still required and which ones are the old ones.

Just deleting all boinc.msi files will also delete the boinc.msi file that the installer is installing. Deleting first before installing the new one, also deletes the boinc.msi file that the installer is using to uninstall BOINC.

This is really an installer thing, not so much a BOINC thing.
The installer that's used is just a 32bit program that holds a database (the actual boinc.msi) filled with the programs that will go into the final directory. The installer doesn't know what kind of programs are in the database.

It will generate a random 32 character name to the end of the C:\Windows\Downloaded Installations\-- path to store a backup of the boinc.msi file in that's used to uninstall BOINC with. It will store this path in the registry at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall

Yep, I know how you're thinking, then it should be easy, right to delete the present BOINC backup? Yep, it can be... but not the old ones as those are unknowns to the installer.

You can manually 'uninstall' the latest BOINC backup file, with the msiexec /uninstall /quiet %systempath%:\Windows\Installer\{difficult number}\Boinc.msi command, but this only removes the latest one, not the previous ones.

I once tried to write a script for this, but embedded it into a full uninstaller, which only worked on half the systems I tried it on. :-/
I no longer dare to link to it.

Anyway, all that aside, eventually BOINC will move towards a new installer, not based on the MSI technology, thus not making these backups. When that is, though? Anyone's guess.

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