Whether or not you believe in the supernatural, there's almost certainly been a moment in your life when you were sure your computer was possessed.
Mine came a few months back. I have a home office, but sometimes when I work late into the evening, I like to move my iMac into the bedroom. That way, after I'm done with work I can just watch Clone Wars reruns in bed until I pass out (don't judge). Anyway, I've just drifted off to sleep this one night, when all of a sudden I'm startled awake by an atrocious noise.
As best I can tell, it's human voices — lots of them — all distorted and babbling at once, like somebody tossed a bunch of English words into a meat grinder and pumped them through a loudspeaker. It's repulsive and terrifying. The babble salad lasts for about ten seconds, long enough for me to get over the paralysing fear that I'm surrounded by ghouls and identify the darkened computer monitor across the room as the source.
I was too scared to get out of bed and to this day have no idea what caused my computer to start speaking in tongues. My best guess is that I left some shitty website open with a bunch of video ads playing, they all buffered while the computer was asleep, and then something — maybe an update or a slight breeze or probably a GHOST — woke my browser up and caused all those buffered human vocalisms to start cranking at once.
Nowadays I'm an absolute nut about shutting down all browsers before I go to bed, and I haven't received any further messages from beyond the veil. Just thinking about those voices still gives me the chills, though. It's probably how Obi Wan felt when Alderaan was destroyed.
But I'm sure you've got a better story. Tell us about the creepiest thing your computer's ever done in the comments below!