how to use ispell in emacs24.3

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hi：
First, I download the the ispell.el.3.7 beta from
     http://www.kdstevens.com/stevens/ispell-page.html ；
Then, I copy it to the directory ~/site-lisp/，and insert the line "(require 
'ispell)" to ~/.emacs file;
but when i start the emacs, there is a warning: 
"Symbol's function definition is void: exec-installed-p " 
So, what are the steps that can make the ispell is avaiable in my emacs24.3 
(OS: windows7) 
thanks!!





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