when formating from windows 10?

Once you have successfully upgraded to windows 10, if you ever want to completely format the hard drive and do a fresh install, simply make a recovery USB from windows 10 now and store it somewhere safe

Control panel> recovery>create a recovery drive

Simply boot off that to do a fresh install. You can dispense with any previous installations DVDs/USBs from windows 7 or 8. Your windows 10 activation details for that hardware will have been stored on Microsoft servers and it will automatically activate when you try a fresh install - you won't even need a product key

Once you have upgraded, your old Windows license is now a Windows 10 License. You do not need to install the old OS and upgrade again if you wipe the drive. Windows 10 has a "reset" function, however, which will wipe out all changes and reinstall Windows 10 without reformatting, which fixes most problems where you'd want to reformat.

windows remembers all previous system if you upgrade. this allows you to downgrade at any time. but a pc also may have a recovery partition in situ. formatting a drive might include you accidentally removing this partition. if so you will lose the ability to install the original OS thats all still there. make sure you have your original installation media(you should have created it at first system start when you purchased it new) if not you might still be able to BOOT to the recovery partition and do it NOW. you will probably need THE OLD SYSTEM FIRST as IT is your QUALIFYING mechanism for installing the UPGRADE to 10. so your formatting exercise is going to take you a long time to fix when you do it.

You should have your windows license ready for reinstallation.