Of course, you'll need a Windows 8 DVD. You can download one here. (It's about 3 GB) Be sure to select the correct version. Both 8.0 and 8.1 are available in both 32 and 64 Bit.
This Evaluation copy of Windows 8 is good for 90 days. In your case, you'll only need the repair functions, and they're good forever.
Note: EVERY Windows 8 user should have a Windows 8 DVD. The best DVD to have is the one available from the manufacturer because it will likely self-activate and have the necessary drivers included. The Evaluation DVDs are a great second choice.
Just be sure to select the option that repairs Windows 8 AND KEEPS YOUR STUFF!
Cheers, PW.
P.S. To download the Windows 8 DVDs, you must have a Microsoft account. (They're free to open if you don't have one already.)