Learn English – What’s the opposite of eye candy

My first suggestion would be a client. I don't agree that it necessarily implies a network connection, but I can see that it could be interpreted this way.

For the operating system I think the association to a network connection is even weaker, so I would definitely call it a "client operating system" to distinguish it from a more server-oriented OS.

Alternatively you could call the computer a "desktop computer" and the OS a "desktop operating system".

I think egosyntonic comes reasonably close:

Egosyntonic is a psychological term referring to behaviors, values, feelings that are in harmony with or acceptable to the needs and goals of the ego, or consistent with one's ideal self-image.

Many personality disorders are considered to be egosyntonic and are, therefore, difficult to treat. Anorexia nervosa, a difficult-to-treat Axis I disorder, is also considered egosyntonic because many of its sufferers deny that they have a problem.

Its counterpart is egodystonic.