How can I reinstall OS X without a valid App Store connection? I have a Mid 2009 MacBook Pro that was used by a "less than responsible" user.  As such, it seems to have become infected with malware that rewrites all SSL certificates.  These certificates cannot be validated by OS X which is what alerted me to the fact that there is a problem.

My issue is that I cannot access the App Store to initiate a reinstallation of OS X.  Since the MacBook Pro can't validate the App Store SSL certificates, nothing loads.

So, I'm stuck.  What steps should I take to reinstall OS X or restore access to the App Store?



Update: Cmd + R on boot doesn't seem to work (perhaps due to the Malware).  Also, the OS version is 10.9.5 (Mavericks).
use Cmd + Option + R to go into internet recovery then follow these steps https://support.apple.com/en-nz/HT201314