Reboot your computer, press F6 before Windows XP loads (hitting it consecutively the moment your computer turns back on would make sure you don't miss it) and you'll be given booting options. Choose Safe Mode, log in as Administer (hopefully you don't possess a ripoff Futureshop/Dell-built computer and left the original Windows install, or you might not be playing Admin until you format). Now, in Admin and Safe Mode you'll be able to make changes to Windows that it'll just have to accept. If it doesn't like it, show it who's boss with FORMAT and a spilt coffee on it's mainboard. Anyways, ignoring that last part, you should be able to make your file replacement(s) and restart normally with the change(s) in effect. Enjoy.
peaCe