In cases where the request is in form "Can you help me with my computer?", not "Fix my computer. Right now." this might work:
Make them fix the problem. Don't touch the keyboard or mouse yourself. Don't answer questions directly, always answer with a question. "Well, why do you think it does that?" ... "Uh-huh, well, how would you attempt to fix it?". Prevent them from doing anything desctructive or misleading. If they're not getting anywhere for a while, then guide them towards the right direction. "Well, that probably has to do with printer... so where would you find the printer settings?" If it's clear they just don't have some some information you have, give it to them - but try not to phrase it as a direct solution or instruction. "Well here it'll probably help you to know that the type of a file is only a clue to what it might contain - a label on a closed box - and you can change it to anything you want but then it'll just stop working because the computer doesn't anymore know what to do with it.", "Great, you now got the wireless connection to work! But with default settings it's still insecure, because encryption has to be enabled separately." Also important: You can say something they are doing is not ok or won't work (in both cases, explain why), but never say whatever they are doing is stupid. Even when it is. Always assume what they are doing is based on insufficient information.
In essence, don't do it for them, rather make it a lesson in computer usage. It's probably very frustrating for them, but you're still helping them so it'll probably be too awkward for them to change the request to "just do it for me". This is because of the social distinction between asking someone for help (acceptable), and someone to do free work for you (not acceptable). But if they do ask, take a deep philosophical voice, and quote "Give a man a fish, feed him for a day - teach a man to fish feed them for a lifetime"...
Upsides:
- They'll think twice before asking you to help again because they know what they'll have to go through.
- You might gain some useful insight to normal computer usage patterns.
- You learn to teach. It's a nice skill to have.
- There's the off chance they might actually learn something too.
Downsides:
- Takes a lot of time. Even to restore a single shortcut.
- Not only frustrating for them - very frustrating for you too. :)