Thor1064: GOG Galaxy is a client like Steam from CD Projekt.
When you using steam you can also verify / repair your installation of Witcher 3. You can find the option in steam when right click on Witcher 3 in gamebrowser (steam).
I didn't know if I understand you right, but why you need to uninstall steam and your other games when only reinstall Witcher 3?
P.S.: Sorry for my english, it's not the best.
Thank you - I emailed Red Projekt and heeded their 'Steam/verify method - it took an hour, but at least I'm up and running again! Thanks for all your help everyone . . . .