@Artista
Windows 10 has a very handy refresh feature. I am not sure if it was there in Windows 8.1.
What it does, is factory resetting the system, something like Android does, but it keeps all user settings. Meaning save games are untouched.
It deletes all installed software however.
Still, the good thing with steam, is that if you install it over the previous folder, it finds all pre-installed games. Of course that suggests that you don't have steam installed on the program files folder.
Even so, before proceeding in such action, it is advisable to copy somewhere else both program files folders, programdata and the user folder. Just to be safe.
Of course all that has as a pre requisite that you have upgraded to Windows 10. In a sense, the formatting of the system as we know it, is gone. Of course you can do a complete system reset and delete everything.
I had a very weird problem with windows update on my tertiary rig, so I did the above procedure, reinstalled steam over its previous folder, the complete visual C redistributable, browsers and utils with Ninite, Antivirus, openAL, XNA, you get the picture and in a couple of hours the system was back to where it was but without the problems.
I am sure most people are familiar with these proceedings, I am just laying it here for anyone that is not.