I would say vista. Many programs would go out of their way to mess with the windows\system32 folder. MS locked most of that out. WinSxS fixed many of the iss

I would say vista. Many programs would go out of their way to mess with the windows\system32 folder. MS locked most of that out. WinSxS fixed many of the issues where you needed to play around in system32. They also added in a huge chunk of 'compatibility' they would ship in the registry of programs they know are messed up with particular combinations of dlls. The downside of winsxs is cleanup. It was not until late in the lifecycle of win7 they added some way to purge old junk out of it.