This is a prank i used to do to people at college and once to myself (It is that good) where a music file is hidden in the start-up folder which would normally be annoying or irritating.
Hope you find this instructive and helpful for an awesome April fools day
P.S The picture of the nuke is used to show how awesome the prank is -_-
Step 1: Fining the File You Want
You can either follow the images or the brief descriptions I gave XD
Step 2: Now for Making the File Work on Startup
Start - All Programs - Right Click Startup - Explore
Step 3: Dragging, Dropping and Finishing
Make sure you save everything that is on the computer just because you want to be nice and shut the computer down.
Next time someone starts the computer they will be shocked at the noise that awaits (for extra effectiveness turn their speakers on full blast and it is unmuted before you have shut the computer down)
Step 4: Enjoy
Hope you enjoy annoying who ever it is you decide to prank.