In theory, all your music should be in the Music folder on your computer. Back that folder up to your choice of external drive, then copy it back to the computer when you have done whatever it is you intend to do.
Note what I said about the location of the files listed in your iTunes library;
- if you used iTunes to copy (rip) music from CD, or bought all digital copies of music from the iTunes Store, all the files will be (or should be) in sub-folders within the Music folder
- if you purchased music from other outlets, you will have used File/Add Folder to Library (or File/Add File to Library) to actually add the music to iTunes, but those files could be anywhere else on your computer
I'm unaware of a "reset" procedure for Windows 10, so I assume that you intend to wipe the computer and reinstall Windows. If so, I will just say that that sounds like a huge amount of work, far more than simply locating stuff that you want to delete.
Consider this:
- if you decide to "reset" your computer (i.e. by reinstalling Windows), you will not have access to your music until you have completed the reset, reloaded iTunes, copied the music files back to your computer and made sure that iTunes not only lists all the music but it can play it too. In addition, if you use Sync with any Apple portable device (iPod, iPhone etc.) with your iTunes Library, when you next go to synchronise, iTunes will want to delete all the music on the device and and then put onto it only what is in your library. So anything that you missed in the copying procedure will be lost. Also, the larger the library, the longer that will take. Oh, and you will also need to check that any Playlists you created in the old iTunes are still in the new one, or you will lose them
- if you were to delete the items that you no longer want, that can be carried out as-and-when you feel like doing it and in the meantime, your iTunes Library remains intact*
* if the items to be deleted include music that's listed in your library, the best advice is to delete the entry from iTunes and if the option to keep or delete the file is offered to you, keeping it will keep the file, deleting it will delete the file itself.
