if you need to do a clean install with the embedded key, you will need the oem w8 installer disk that matches your stock OS. use slmgr /dlv to determine it, and then find the matching one according to the version (core, single language, etc. it should be of oem channel).
alternatively, you can get to the clean state of your stock OS by resetting everything (using the option that 'remove everything and reinstall your windows')
before you do anything, please create a recovery usb with asus backtracker.