I detect a lot of resentment in your post that could all be avoided by just saying "No, sorry, I can't fix your computer."
So I should lie to them? Because that's what I would be doing. The truth is that I can fix their computer. To make that a truthful statement it would have to be, "No, sorry, I won't fix your computer." Has a bit of a different ring to it, doesn't it?
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.
I would say "yeah, I could probably fix it, but it'll take me at least a few hours and even then it's not certain. You're better off taking it somewhere professionally where they can properly backup your data and be held accountable. These are the guys I'd recommend."
No offense, but it sounds more like you want to fix it. Not that there's anything wrong with that of course, it's your prerogative.
By agreeing to fix their problem, you are now held liable at least in their mind. You are that "professional that can safely backup their data..." or whatever else they think you are. Surely if you can handle a big corp IT department you can take all that proprietary software home (piracy) and solve their problem?
Now that I've left IT directly into software development, I get to play the "Hey I'm just as illiterate as you!" card which is at least partially true. I don't have expensive software or even take proper backups of my extremely important data I wouldn't store in the cloud. Would you want someone like me touching your computer? You shouldn't.
I've done everything I can to be unavailable. It's a choice and one I proudly make. I just don't have the free time for charity any more no matter who you are to me unless I'm actively seeking some form of community service.
I always say since they basically want me to work for them I need to charge them like any other client. Since computer support is not my forte it would probably cost a lot.
So I should lie to them? Because that's what I would be doing. The truth is that I can fix their computer. To make that a truthful statement it would have to be, "No, sorry, I won't fix your computer." Has a bit of a different ring to it, doesn't it?