When a No-Op is OK

In Windows 95's Explorer, clicking "Up" while viewing My Computer produces a modal error dialog that says something like "You have reached the root" and says that you can pick another root destination from the Address drop-down if you want. (Back then, the desktop could not be viewed as a folder in an Explorer window.) This behavior is super annoying because it requires me to either move the mouse to the center of the screen to dismiss the dialog or move my hand from the mouse to the keyboard to hit the Enter key.

This is a situation when it's OK to just not do anything. I understand that you can't go up any farther from My Computer. (Or, today, the desktop.) The best option would be to just disable the Up button (which Windows XP might do, I don't remember) when it is not possible to go up. If calculating whether a button would do anything is expensive, I think it's OK just to do nothing, provided it's sufficiently obvious that the program is not in a state where it can do that.