php.ini include_path

The php.ini file will have include_path already in it, but commented out, that is where you should put it, by uncommenting it. It also has examples for windows. It will look like this, just remove the semicolon preceding "include_path"

; Windows: "\path1;\path2"
;include_path = ".;c:\php\includes"

This all depends on what you are trying to accomplish. Personally, I don't edit the php.ini file directly for setting include_paths, rather I use the following construct, in code:

// This will append whichever path you would like to the current include path
// PHP is smart enough to convert / with \ if on a Windows box
// If not you can replace / with DIRECTORY_SEPARATOR
set_include_path(get_include_path() . PATH_SEPARATOR . 'my/custom/path');

-- Edit --

Chances are there may be multiple copies of php.ini on your system, and that you are not editing the one that is being used by PHP.