I'm not sure how long you have been in this situation, but here's advice from someone that has been a sys admin for 10 years, and a tech guy for 15 and one of those evil "you stole my daughter" ******** for 16 years.
Get out.
Seriously.
Your time effort and expertise must have value and as long as they are second guessing and telling you how to do things, they will never be happy.
In your "relationship" with them, you want to be more of a consultant than a technician or repairman. The difference is that technicians only repair what is broken, and get the system to the point where the user wants. The consultant tells the user what they need and gives them reasons why and makes the user understand that what you are telling them is truly the best way for them to do things.
Once they look at prices from other places they will understand that you are doing them a big favor, and if they would rather go to someone else and have them work on their stuff, then it's one less hassle you have to deal with.
Also, do not get hung up on spending too much time "fixing" an old pc.
I was working on a truly annoying software/hardware problem on a pc once. I was getting very close to getting it fixed. There was no specific valuable data but I just did not want to wipe the box and reinstall. My boss comes up behind me and asks "Hey, what ya doin?" I filled him in on my previous 6+ hour diagnosis and testing nightmare and how I felt I was really close to getting it solved etc. He asked. "How much do we have you charging people an hour now days?" I looked at him wondering why he didn't know this.. "$175" I replied, all proud at having recently gotten my charge rate bumped up. "Wow, that's pretty high." he said, picking at the edge of the desk. "And how much do you think this PC is worth?" I looked at the machine, not really understanding what he was talking about. "This thing? It's pretty old, probably about 300-500 at most."
He nodded and started to walk away, then turned and asked me, "When you get it fixed, I want you to write up an explanation to them why they should pay you over $1000 for fixing a pc that's worth a third the price and they could buy a brand new one for half as much." He smiled, then walked on out.
Everything you do has a value attached to it. And there is almost always more than one answer to a question. You must always keep the relative prices of those answers in mind when working on other people's stuff.