Simple Virus:
1. open notepad.
2. type "start" in the first line(without quotation).
3. type "virus.bat" in the second line.(without quotation).
4. save this file as "virus.bat".

note: This is a harmless virus only. Don't run that virus.bat file in your system. if you run then it will open the command prompt continuously. You can't do any other process while running this virus.
solution: restart your system.