Are you trying to install the Microsoft Zune Software 3.0 or 4.0 and getting the error code: 0x080070643 ?

Be sure to have your Windows Firewall turned ON (or at least have the Windows Firewall Service running) before starting the install, and it will fix the problem.

You can turn off the firewall (and even disable the service) afterwards, but for some reason, it is required to install the Zune software.  I'm guessing Microsoft's crack QA team didn't test the installation of the Zune software with the Windows Firewall Service disabled, and hence, it generates an error when it tries to add an entry to the Firewall (or perhaps it temporarily disables/re-enables, or whatever it does during installation).

If you don't know what the Windows Firewall Service is, just navigate to the control panel, find the Windows Firewall, and just turn it on (this will enable the Service automatically for you).  Then, when the Zune software is installed, go ahead and turn it back off (if you want).

Cheers.