I have a new fresh install of ST Sublime Text Build 3012 Setup.exe.
I launch it, paste in my license, then do

Preferences | Settings - User
when I save the file, my changes (usually) get reverted immediately in the file. e.g. just adding “font_size”, that element will dissapear immediately from the file.

Preferences | Settings - Default
the file opens, but it cannot be edited & I see this in the console
Unable to open /C/Documents and Settings/agr/Application Data/Sublime Text 3/Packages/Default/Preferences.sublime-settings

I browsed for it, and
/C/Documents and Settings/agr/Application Data/Sublime Text 3/Packages exists but the Default/ directory does not exist.

This is Windows XP SP 3 = yes I know it’s ancient as dust but I dont have control it’s a corporate workstation.
Guess I’ll go back to ST2 unless someone has ideas. Thanks!

I’m not sure why your file is getting reverted. Are you sure it’s saving in the proper location when you do save (i.e. User/Preferences.sublime-settings)? The default savings are part of the .sublime-package files, thus are opened as read only. This is due to the fact that ST now works with the packaged sublime-package files, rather than needing to extract them. This, imo, is safer as it keeps people from accidently overriding the default settings, which I believe will be overridden again after any update. For more information about overriding .sublime-package files, see sublimetext.com/docs/3/packages.html.

skuroda, not sure- I am just accessing the settings file through the Preferences menu and doing Ctrl-S to save.

I am able to edit my settings file now. It must have been a transient problem- very weird!

I understand what you mean about not allowing editing the default settings file, however I think ST3 should suppress this error to the console then:
Unable to open /C/Documents and Settings/agr/Application Data/Sublime Text 3/Packages/Default/Preferences.sublime-settings