Regarding which programs/files to keep, I'd like to mention this. If you have any paid-for programs, ham or not, back them up first, then uninstall them. Uninstalling them will give you your activation back (with most programs). If the program has an export/import function, you won't need to back up the program, just the data, and a flash drive might be a good chose for that, or use an external hard drive.
Before uninstalling, you could use something like Aomei backup, Easus Todo backup, or another free backup program and back up to an external hard drive. When you've reinstalled your programs, you can use the backup program to restore the folders of the programs to make them like they were before.
Don't forget that some programs use the apps/data folder to store some things instead of putting them in the program files folder. I wish Windows hadn't separated program files into two areas, but they have. Some programs let you avoid this, like my email program. For my email, I copy everything to a flash drive then restore to the root folder (C:\). This came in handy when I added my email program to a new laptop.
Don't forget the usual things to back up, like your data folders, and if you need to have your browser's bookmarks, either export them then later import them, or back up your browser's folders (including the apps/data folder).
As for how to drill down to the apps/data folder, I'll leave that for someone else to explain if you need that information.