'Yeah, it's to a friend -- sorry, it's kind of personal'. There. You've just told him to stop asking about it and been polite at the same time. Even if he shouldn't have been asking, I personally find it really grating when people just don't answer rather than coming out and saying they don't want to talk right now/ don't want to discuss something in particular. Also, if you actually say expressly that it's not up for discussion, he can't really come back the next day and ask you about it. Or if he does, then you are justified in just telling him to mind his own business.
Personally if I have an intensely private matter to deal with I don't do it in public where everyone can see my screen. That way no-one asks me about it.
... and the ones that won't