I would make something that will buffer your videos a tad bit slower. For example you would be watching a video and it freezes for 2 seconds because it has to buffer.

One that slowly lowers your PCs volume down to 0 every 20 minutes.

Change your resolution to like 320x240.

messing with your headphone EQ every 22-28 minutes

A orange dancing on the screen, distracting you while I take all you digital cookies and bank information

Restart your device every 10 minutes

every 30 minutes a small voice clip plays of tony soprano saying "gabagool"

Like 2 times a day replaces a letter or number you type with a random one.

One that forces "their" to be spelled "they're" and vice-versa. Also "too" and "to"
So that any attempt to post anywhere will always be hounded by grammar police.

Selecting Power > Shut Down results in a system notification 'are you sure' audio of Denholm Reynholm from The IT Crowd and clicking 'yes' results in the same notification repeatedly so the only fix is to pull the power cord.

Replace the loading symbol with a little Rick Astley dancing.

Nah, overclocking the person's computer is too finicky for a mild inconvenience. Get it wrong and you might break the system, which is a major inconvenience or improve system performance which is a net benefit.

Also it can't be too frequent or impactful or else it goes from being mildly inconvenient to moderately annoying or even a major problem. Don't like messing with people's volume either because if they're wearing headphones you might deafen them. Well, maybe lowering it is okay, but never raising it.

I think I'm just going to switch your desktop pattern to blue Mac O.S. cats.[x.com] You might not even notice until you exit fullscreen, and since it is meant to be a desktop background in the first place you can probably see your icons.

Fixing it only requires you to change that wallpaper back, and if you don't know how to do that it's still not a big deal.

Changes all your icons to Gaben.

One that changes your mouse scroll speed to 1

I would port the Newton virus to Windows and Linux.
https://www.youtube.com/watch?v=xuLdJQY-XNI

Alternatively, I would make a virus that randomly switches the left and right mouse button.