Just recently my computer did this for the second time in a long time, and I decided to post it for some help.
My computer sometimes, when idle, suddenly goes into what seems like a sleepmode(black screen) but will not respond to keyboard/mouse input. The catch is; it sends the fans into a crazy frenzy. I think my case fan was built for 3500~4000 RPM at full speed but was easily doing 6000.
Since I downloaded/installed ZoneAlarm(freeware kind) last night, the computer has been randomly blakening the monitor and speeding up fans/HD when it's idle as if it were rebooting, but instantly recovers back to the desktop.
Is this a BIOS problem? I haven't had this computer a year yet, and sleepmode/screensaver/hibernation is disabled.