So, your Windows currently looks cruddy and old?
If so, then you probably have deleted it, so just (likely from safe mode) replace the files; simple as. You can get them easily online. As long as it's the same OS, I can't see why it wouldn't work. Alternatively, just use a System Restore.
BTW, this is a software thing, and not a hardware thing, so things like this are probably better left for the Software forum.
(welcome to the site, BTW)