My machine was suffering from lags and some lockups; it occurred just that one time. Firefox would initiate but the window would not open.

The PC shut down by "NT AUTHORITY/SYSTEM" prompt; so I had no control over what occurred. I did remove PC Tool's Threatfire because I had a nudge that may have been the problem.

At the moment the computer is running normally; a screenshot is attached.
Does anybody have an idea on why this would occur?