You can tell them whatever you want. The point is that you're saying, "No".

I don't have time to be fixing friends and acquaintances computers and I have no qualms about telling them that. I've never had anyone give me a hard time about it. A lot of people assume that because you are knowledgeable that you may want to fix it, a point of view which is only reinforced when you gladly take their broken computer and return it to them repaired without putting up a fuss about it.