You will need to reinstall the file from an OS installation disc. You may have to reinstall the entire OS. If you need to, you should be able to keep any documents and other data you have on the computer, so long as you are reinstalling the same OS. If you want to upgrade to a different one, however, I would recommend that you use another device to boot your computer (a cd, external hard drive, or flash drive with the same OS you have on you computer), and copy all the desired data from you computer onto another storage device before upgrading. It is complicated to just replace the file manually, and it still requires a resource with the file on it (such as the installation disc), so your best would be to just reinstall the OS, or changing the OS.