Security Update Crashes Macbook

I installed the security update yesterday, and since then, I have not been able to get past my login screen. I get to the login screen, type in my password and the loading bar makes it about 3/4 of the way before something pops up on the left side of my screen for a second or two that looks like command prompt dialog with headings about "Threads", then the screen freezes and the computer restarts to the flashing gray folder with the question mark. The command-R combination does not work because the Apple logo always pops up when I turn my computer on. Once I shut my computer down, it restarts saying that my computer restarted because of a problem and the cycle repeats.