Now am going to explain one harmful Trojan. one C# Programmer can write this program. so didnt provide the source code of this program. in C#.net there is one namespace System.IO, with the help of mentioned namespace we can get all details of Directory structure and file name folder name etc of one system or one computer. you can get programming help for directory and drive details from MSDN.
How to Do?
just find all local drives and files then u delete all files in drive programmatically. this is most harmful. so dare to code and Debug.