The factory installation files are available on your computer, built-in. Most computers don’t come with a recovery CD anymore, it’s just built in to the computer on the hard drive. Every computer manufacturer has different ways of accessing it (usually it’s pressing a special key when you start the computer, but my Sony computer has a special pin-hole to press with a paperclip to start the recovery process). If it’s not built in, and you can’t use the Refresh/Reset options above because you don’t have the media, you’ll have to call your computer manufacturer to request a recovery CD/drive be sent to you. Some companies charge for this, but I had great luck with Dell, who shipped me a recovery CD by FedEx overnight delivery.
If you want, you can also get them here; Create installation mediaforWindows 8 (but it won't come with the drivers you'll need, so you should get it from your manufacturer or use the built-in option instead).
Here's how to use the built-in option: http://cmdrkeene.com/quickly-repair-pc-problems (use option #3).