This will freak out your friends or whoever you do it to! They will think that their hard drive will get fried. It is completely reversable.
Virus Prank 2: https://www.instructables.com/id/Virus-Prank-2/
Step 1: Run the VIRUS
In the start menu, there should be something that says "Run". Click on that and type in "VIRUS" and press OK. An error message should come up with the title "VIRUS".
Step 2: Paint
Now do "Prnt Scrn" (Print Screen) and paste it into MS Paint. Now click on the tool that copies a color and right click on the grey part of the error message. Now erase all of the text.
Step 3: Write New Words
With the text being "Times New Romans" and the font size 8, write a new error message in all CAPS. I wrote this: "WARNING! A VIRUS HAS BEEN DETECTED! SHUT YOUR COMPUTER OFF RIGHT NOW AND TAKE IT TO A PROFESSIONAL TO PREVENT YOUR COMPUTER FROM NEVER WORKING AGAIN!" You can write something different if you want.
Step 4: Make It the Background
Now all you need to do is make it the background of the computer. So the next time someone turns it on, they will freak out!