[chupasaurus]

You've missed "Install Steam somewhere inside separate user's home directory" to not dealing with UAC on each update.

I moved to separate Wintendo box which is the best solution.

[ChrisSD]

As far as I'm aware, Steam makes its own folder writeable by everybody. Besides, you should get an error instead of the UAC if you're running as a non-admin account.

But yes, separate hardware is safer.