To get an installation USB, you will need the Windows 8.1 Pro .iso file or the DVD (but if you have the DVD, what's the point?).
If you don't have it, you can make a recovery USB stick. To make one:
1. Open "Control Panel". (Desktop UI version!)
2. Search "recovery".
3. Click on "Create a recovery drive." under System.
4. Follow the instructions.
5. DONE!