I have the original disk I used to install it (Windows 8.1 64 Bit OEM).
Remember, this will delete ALL personal files; everything.
Also, confirm there is ONE hard drive installed in this computer and that ONE hard drive has the operating system you want to delete. If yes, move on.
Boot from the disc and go through the menus as if your installating until you come across "Upgrade" or "Custom" installation. Select Custom installation, and click the circled object shown int his photo:
After that, select the first partition on the list and press delete. Do the same with all the other partitions until it
just shows something like this:
698GBs Unallocated
Once you're done shut down the computer, restart it, eject the disc tray, take disc out, shut down, done.
Then you ought to be good to go.