The stupidest program I have written has to be a program I wrote to annoy someone in the programming class before mine in high school. I knew this character always used the same computer that I did and that his class was always at the same time. I downloaded a C++ TSR library known as tsr100je.zip from SimTel and coded up a program that would stay resident and then launch at the appointed time. I then modified COMMAND.COM on the computer so it load a different file at startup than AUTOEXEC.BAT, and stuck my TSR into the new AUTOEXEC.BAT. I turned off my computer and then left.
Apparently, the program worked and it really perplexed its itended target and my programming teacher. She had me check the computer for virii when I was in class, so I removed the program from the new AUTOEXEC.BAT and that was the end of that.
Any way, the program would start off pretty benign: it would print random characters on the screen at random intervals. Then the real fireworks began. I had just learned how to change the colors in the color pallete in text mode, so I had the screen appear to fade to black. For effect, I had the program fade the monitor in and out for awhile using random colors. Finally, I disabled Ctrl+Alt+Delete and printed some random messages on the screen.
It was fun to write, but boy was it useless.