Through XAMPP you probably set up some folder as "webroot" or "htdocs", or whatever they call it. Place your mootools-core.js somewhere inside that folder, and use <script src="/path/to/mootools-core.js"> in the <head>-section of every page. The path you use as src-attribute is relative to that webroot/htdocs folder.
So say you put the mootools file in: C:\path\to\htdocs\js\mootools-core.js The src-attribute will be "/js/mootools-core.js".
