Anti-virus software - If you have a real-time virus scanner enabled, and it happens to be scanning the file in %windir%\installer at the same time as setup is trying to open the file and install from it, you will get this type of error. This is generally a timing issue, and in this case re-running setup will fix it. Alternatively, you can disable real-time virus scanning during installation, but if you do this please be aware of the security implications of doing so and either install while not connected to the network, or if that is not possible, make sure to immediately re-enable the virus scanner after setup completes.