Can you be a little more detailed as to what problem you are having and what message you get? The immediate answer to your question is that you can't download it free, you have to pay for a Microsoft OS, but we can probably be more helpful than that.
Do you have a machine that used to boot and does not boot now, and are you getting a message that that particular file is "missing?" Most of the time it isn't actually missing, but the boot sequence is corrupt. There is even a known issue where a bad keyboard cable can lead to the message that this file is missing or corrupt.
Please provide information about the kind of computer, what OS is installed, and what the problem is. Do you get a message about this file when the boot fails or only when you try to read your mail? The clearer we are about the circumstances, the more likely we are to be able to help you.
And now, my un-patented one-size-fits-all solution which you absolutely should not try unless you have a backup of all the data that you ever want to see again. Read that warning again: I do not guarantee that trying this will not destroy all the data on your drive, so be sure that you have backups.
My best guess is that you need to boot from the installation media and select the option to repair the installation.