Whenever you reinstall windows, you will need your license key. As long as it was the same key that you or your manufacturer used to activate your copy of windows, it will always work so long as you don't replace your motherboard or your hard drive. For help with finding your license key, I recommend this program:
http://www.nirsoft.net/utils/product_cd_key_viewer.html
Make sure to record your key down. As long as you download and install the same edition of windows, you should be good to go as the key displayed by the program is linked to your computer.