The CDKEY can be retrieved with the MAGIC JELLYBEAN FINDER (see google.)
Remember that what I'm going to share is good enough for most but those that are paranoid just won't be enough.
The simplest method is to create TWO (this is important to have two) LOCAL LOGIN ADMIN accounts. When you are ready to dust off your old account and its files you can log into one of these and then remove your old account. When you do that Windows will ask if you want to keep the files, you answer you want to dump those files and you have cleaned out most of your junk.
If you want to uninstall apps, then do that.
A final step would be to use CCLEANER (see google) to wipe empty space which is noted at https://www.ccleaner.com/docs/ccleaner/using-ccleaner/wiping-free-disk-space
Yes, there may be a few remnants but it's 99% clean and good to go.