Change the start menu search so instead of finding local applications and files, it searches the internet.
Would be even funnier if it used the worst search engine available.
Oh wait…

I shall write a virus that makes the computer play the “USB device detached” sound followed shortly by the “USB device attached” sound. Dee doo. Doo dee. Just that. three or four times a week.

Alt+Tab goes to a random window instead of being in the order of recency

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.

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.

I’ll call it Windows 11.

A virus that changes the font every time you go to a new page or hit refresh

I had a boss that wasn’t exactly technical. I wrote a power shell program that would randomly every 5-30 minutes give a pop-up that said “good job”, which he always said regardless of what was going on. Placed it in his startup folder on his machine. I thought he would figure it out and tell me to knock it off… Well I forgot about it, 9 months later during my annual performance review it popped up while I was looking at his screen. He apologized and just alt tabbed it away.

I offered to take a look and see if I couldn’t stop it, and he said yes and then walked away to take a break. I then deleted the script I put on there. He gave me extra performance points (meaning a higher pay raise.)

There’s a 1 in 50 chance that any copy text command cuts the text instead, and vice-versa.

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.

The version I recall was once if those Flash animations with a cute squirrel or whatever saying something… but it was really quiet so you’d need to turn up the volume to hear. Then partway through it changed to sex stuff and blasted out in a voice like a monster truck announcer

“anal sex dot com, all anal, all the time!”

When you click the mouse button, the pointer position moves up to 10px in a random direction before applying the click.

I wrote a simple script once that ran in the background and all it did was toggle the state of the caps lock key every 30 minutes. I set it up on a co-worker’s computer as a scheduled task for an April Fools prank one year. I thought for sure he’d figure it out pretty quickly, but by mid-day, he had completely disassembled his keyboard, convinced the button was getting stuck due to gunk buildup. Eventually I ended up just disabling the task so he thought he had managed to fix it himself.

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.

Oh, I have a seemingly harmless idea so evil, it will ruin the internet forever.
I will make it so every time you open any website, there will be a popup with a question that asks you to invade your privacy, and you can allow it to do so with one click, but you will have to dig through menus if you want to avoid it. Then, after some seconds, another popup will appear, asking you to create a login, no matter what you do. Then, it randomly will ask you to share your location. Yes, with a popup again. Then, just as you thought you’re done, another window will open, grabbing your focus, which will demand you talk to a chatbot, and you can’t close this one, only slightly minimize it.