How does a long time WinXP user clean up a Win7 desktop with Win10
installed without losing Windows 10 in the process?

My main question right now is how to proceed to do two things:
1. Wipe out everything not needed
2. But keep the Win10 OS intact

I am a long time Windows XP user who just bought an HP desktop at a yard
sale that has stickers for Windows 7 but that has Windows 10 installed
(complete details to be placed in a followup post).

The desktop came with a single user with admin privileges so I created a
new user since the original user is already using up all but 380GB of the
930GB HDD.

If I had Windows 10 disks, I would start with a fresh format and install of
Windows 10 - but I don't have any media whatsoever and the original media
would have been Windows 7 anyway.

Do I need to save some kind of serial number before I format and start
over?

My main question right now is how to proceed to do two things:
1. Wipe out everything not needed
2. But keep the Win10 OS intact