It's funny, I am the pc guru around my family and friends and I'm pretty consistantly asked to help "fix" their computers.
Sometimes I wish they would call as soon as the problem shows up, but more often than not its when they have "broken" it beyond hope. Usually I try for about 2 hours or as mysterea said, "until it pisses me off". At that point its a clean wipe and a reinstall of some sort, or I take them to the store and make them buy a new machine. I usually start off these conversations with "you know if I help you, theres a good chance you will lose whatever you have on your machine, right?".
Sometimes though you have to bite the bullet and go with a new machine. My dads last pc was like this, and it was one I built for him. But he had hardware issues that culminated into a bad motherboard and I got tired of fighting to fix his outdated stuff. When the magic smoke puffed out of the ram, we went to the store that night and bought one off the shelf for him.
And I always remind people to make mulitple copys of their most important stuff, always and often. You never know when that HD is going to crap out and you lost everything you have. Or that virus decides to do a full format and your done. Whats funny is that this never happens to me, I'm not sure why. then again, noone except my wife touches my pc, and very sparingly at that.
The kids get brand new dells, one a low end, one top of the line and still manage to screw them up within a week.....unbelievable!