I don't know Windows scripting, but that looks like something you'd have to put in a file and call for it to work, instead of just running that on the command line. You can do the same thing on Unix systems. Writing

  $0|$0

on a file and running it should also work as a fork bomb.

The conceptually same thing will also work on bourne shell. But as in the Windows case only as a script stored in file, the original with inline function definition is self-contained oneliner.