Here is a way on how to create a simple batch file virus

Write it in a notepad
Cycle a message in your friend's computer(code is given below)
Type :
Code:

@ECHO off
:Begin
msg * Hi
msg * Are you having fun?
msg * I am!
msg * Lets have fun together!
msg * Because you have been o-w-n-e-d
GOTO BEGIN

Save it as "Anything.BAT" and send it.