Depends. How is your copy of Windows licensed?
If you bought it in a box, it's retail. You will need the key to reinstall. See if you can find the box.
If it came with the machine, and the machine is from a reputable vendor (Dell, HP, Asus, etc), then it's an OEM license. If you get the right media, it'll actually activate automatically as soon as you reinstall. Also, your license key should be on a sticker somewhere on the case--look for a rainbowy sticker with holographic Microsoft logos. For media, your best bet (if you didn't keep the system recovery disc that should have come with the machine) would probably be to get ahold of the manufacturer--they'll usually send it to you for a couple bucks + S&H.
If (and this is relatively unlikely), it's licensed with Open license, you'll need the license key there, too. Contact whoever sold you Windows, they should have a login to the Microsoft partner site to download ISOs for you.
...or you could try the above-mentioned media creator, if you have a 'net connection where a 4GB download is feasible. Can't speak to the efficacy of that, I've never needed it--my office is lousy with copies of every CD and ISO for installing various versions and editions and activation channels of Windows that you can think of. Perks of being an IT guy.
EDIT--if you don't know your license key and can't find it, before you go blowing away your installation, try Magical Jelly Bean--I've actually had a good bit of luck with that thing--it crawls your registry hive, finds the key, and decrypts it for you. Don't know that I've needed to try it on Win8 yet--usually when it comes to that it's because someone lost their key sometime in the 5 years since they bought their rig.
SOURCE--have personally installed Windows more times than I care to think about.