When Steam is runing ok, make backup1.reg & backup2.reg of both those Keys(with regedit.exe):
[HKEY_CURRENT_USER\Software\Valve]
[HKEY_LOCAL_MACHINE\SOFTWARE\Valve]
And make another delete.reg file for valve reg deletion with this content(with notepad.exe/ + change ext. to reg):
Windows Registry Editor Version 5.00
[-HKEY_CURRENT_USER\Software\Valve]
[-HKEY_LOCAL_MACHINE\SOFTWARE\Valve]
delete valve-reg(run delete.reg) before switching css versions, + when switching from nosteam to steam run backup.regs after delete.reg.