Three options come to mind.

A virus that adjusts your mouse sensitivity by like 5% every time you unlock your computer. Just enough that existing muscle memory is off, so you either have to adjust to the change or change it back every time.

A virus that installs and/or sets a similar but not quite right keyboard layout, and swaps to it randomly few boots. For example, setting the keyboard to Canadian Multilingual Standard instead of US English, where its only some of the punctuation keys that are changed.

A virus that randomly pops up a terminal window and outputs suspicious-looking text, and closes itself before the user has time to read it.

When I worked at an MSP in early 2000s they would “prank” new hires with a site that did this. It would keep creating popups so you couldn’t close them. That stopped after customers heard it. More horrid graphical “pranks” replaced it of course.

At first it all seems normal, every now and then a random sound effect is replaced by ominous hooting. Every hour, on the hour, a green owl flashes on the screen for a frame or two, it’s eyes boring into you before vanishing. Once every 50 or so times it pumps your volume up, selects speakers as output and let’s loose a screeching hoot. Random popups slowly ramping up “Restart your streak today”, “Where did you go?”, “Duo misses you”. At first just once or twice a day, but steadily increasing in intensity till it’s one every 23 seconds.

A virus that changes Windows’ sticky keys to only requiring two taps of the shift key.

I don’t remember the details but my mom’s first cousin called me once to fix her computer when I was a teenager.

No matter what she typed it came out as, “I AM FUCKING GAY!”

Seems like all I had to do was type “stop”.

Uhh, it was “Hey everybody, I’m looking at gay porno!” and then it proceeded to spawn hundreds of internet exploder windows pointing at goatse.cx. Thank God I was at home, alone, when it got me.

I had a friend who sent me a “Y2K fix” program back in '99. Said it would patch the error so I’d be safe. When I ran it, it swapped the letters Y and K on my keyboard.

I was in college during the years leading up to y2k and supported myself at the time getting IT infrastructure ready. Some friends and I decided to write a “virus” that, on bootup, checks to see if the current date is in the first week of January 2000 and if it is and a backup of the fonts is not found (so it’ll only run once) then it’ll back up your fonts and alter the originals to replace the y character with the k. This affected everything system wide.

That created more chaos than anticipated.

While the computer is on, at random intervals, it shall play the sound of a smoke alarm’s low battery beep using audio technology to make it sound like it’s coming from somewhere else in your house.

There was a guy in my dorm who really didn’t like his roommate. Really, really didn’t like him. This was in the early aughts.

So one day he goes on his roommate’s computer and puts a text file in his startup folder. The file says, “Your computer has been infected by the Snood virus!”

For context, Snood was a free video game people downloaded in the early aughts. Basically the same as Bust-A-Move, which probably doesn’t clarify anything if you didn’t already know what Snood is.

Anyway: “Your computer has been infected by the Snood virus! If you don’t score [extremely difficult but not completely unrealistic high score] points, all of your files will be deleted!”

He laughed to himself and promptly forgot about it.

Weeks later, the roommate is on his computer in the middle of the night.

“What are you doing up? Go to bed.”

“I can’t. It’s this stupid Snood virus.”

How harmless are we talking? I’m thinking of one that randomly “locks” the next file you try to open that was last accessed over 30 days ago. It prompts you with “File locked by last user, please enter username/email and password”

No matter what you enter, it unlocks the file.

Then the next time it triggers, it prompts you again, but blocks you from using the same username and password.

Rinse and repeat while the user keeps giving you all their user names and passwords over time.

When I was in high school in the 90s a group of us in computer class made a ‘virus’ that would launch the hamster dance website in all of the classroom computers randomly. We had to put it on a diskette and install it manually on each computer but at the time none of the computers even had antivirus so the school had to reformat them to remove it.

RedReader my beloved 😢

Not so harmless to the guy living in Iran that got stoned.

back when I was in school someone wrote a script that just openened the optical drive at random intervals and put it in the Autostart of every PC in the Comouter room