I had something similar happen the first night I was playing Rift. I had played for a few hours, and let my wife sit and play with the character creation screen while I was getting ready for work. For some reason my computer just did a total power off while she was dinking around with a character, our power didn't flicker and the power strip is placed in a manner where the button can't be accidently turned off. Computer rebooted fine, and it hasn't done this since.
Also, I had a problem for about 3-4 months where my computer would randomly go into shutdown mode. It was never consistant, sometimes it'd happen 2-4 times aday, sometimes I'd go a week without it happening. It'd close every window I had open and go into Windows shutdown (Really sucked the night I was tanking LK in phase 3 and this happened!]. Ironically enough I discovered it was my wireless keyboard causing the issue, either from outside interference on the same frequency, or a fault within the keyboard/receiver itself.
Edited, Mar 18th 2011 5:37am by SilverGryphon