Reformatting a PC that is running a free upgrade Windows 10 Pro

I upgraded to Win 10 about a year ago on Microsoft's free upgrade program. I get the impression that my PC is now full of all sorts of unwanted crud - it has slowed to a crawl, and no amount of scans, cleaners, etc seem to help.
I want to completely scrub the drives (I have two full data backups) and reload the OS and software. I have all the software, but am not sure that the ISO I made when upgrading to Win 10 will still work if I load it and try to activate it. I have the product ID, but I guess that is generic to Win 10 Pro, and not specific to my license. With the free upgrade program, I never of course had a Product Key. With the Free Upgrade program now over, what will happen if I reformat and reload Win 10? Will I be able to activate it?

Yes.

Activate Windows 10

Web search on "digital entitlement". Microsoft's activation servers will automatically activate new Win 10 install on the same hardware.

I would start with a Windows 10 USB flash drive or DVD created from the most recent ISO available:
Windows 10 ISO Download - Windows 10 Forums

USB Flash Drive - Create to Install Windows 10 - Windows 10 Forums

Windows 10 - Clean Install - Windows 10 Forums

Boot the computer from the flash drive or DVD. When asked for a product key, select the link at the bottom for "I don't have a product key". Select the custom install option when asked. Select Windows 10 Pro when asked. After selecting the custom install option you will be presented with a list of drives and partitions. Select the drive you want to install Windows to (probably drive 0) and delete every partition on that drive. This will erase everything on that drive, so make sure you back up what you want to keep first. When the drive is just one big unallocated space with no partitions left on it, select the unallocated space and click next. That will let Windows setup create the partitions it wants to and install Windows 10.

Activation over the internet will be automatic based upon the digital entitlement stored on Microsoft activation servers when the computer was upgraded to Windows 10 based upon the hardware ID of the computer.

Thank you bobkn and Navy LCDR. Your advice gratefully received.

bobkn said:

Yes.

Activate Windows 10

Web search on "digital entitlement". Microsoft's activation servers will automatically activate new Win 10 install on the same hardware.