MacBook 13-inch Early 2008, Lion 10.7.4, latest System Updates, 4 GB RAM, 500 GB drive I recently tried to open an MKV under QuickTime on this machine. It apparently has Perian installed from a previous version of the OS, and when QT attempted to open the file, QT crashed. I force-quit everything, but this did not help, so I force-booted. After rebooting, the machine entered a sort of "reboot loop". The machine would start up and then automatically log in to the main user account. Several Word documents appear, because the wife tends to leave everything open and running at all times. Shortly after, the screen turns translucent white, and then it all happens again, and again. I stopped this loop by rebooting and Safe Mode. In this case you are presented with a login screen, and using any of the other user accounts on the machine works fine. However, when you try to log into that account, it crashes back to the login screen very quickly. I can find various pages talking about the "login loop", but the actual solution seems difficult to understand. It appears there may be damage to a cache file, but how to address this from another login is not widely reported - I don't have access to the file in question.