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Some computers will allow you to DISABLE the Fn key.
But this is done in BIOS, and only certain models of computers will do it.
So, what is your Make and Model of computer?
you would have to write a custom batch script to REVERSE how the keyboard is originally designed. Not the easiest task.
However i do beleive there is software that will let you customize your keyboard in just about every thinkable combination there is.
Let me see if i can find it.