When to format

It is hard to know, while working on a computer, when to give up and format the hard drive.

I worked on a customer's computer recently and spent several hours without coming to a successful conclusion. Thinking about it now, I realize that it would have been faster, and more satisfactory to the customer, to, once the normal methods of problem-solving had been exhausted, save all of the customer's irreplaceable data and format the hard drive and then re-install Windows.
The problem I faced was that Windows was functioning normally, mostly, and the Internet connection was stable, mostly. Even with the few minor viruses gotten rid of, the problem lingered. With this type of situation, it is easy to think "Well, it must be a virus, or spyware, or this, or that..." and keep looking for something concrete that you can say "This is the problem!" and once you get rid of that thing, all will be right with the computer.
But, the problem with that is you get stuck in a rut and can't think of other solutions to the problem. Which is what happened to me, leading to customer dissatisfaction.

So, now I think that if I have spent two hours or so diagnosing and scanning for the most common resolutions to the problem and have not found a solution, then it is probably time to backup all irreplaceable data (all the stuff you dont want to lose) and consider re-installing Windows(by formatting the hard drive).

Different people, geeks, repairmen, technicians, will all have different ideas about this, but I think this is the most workable answer to the question "Should I keep trying to fix the problem or is it faster to format the hard drive (and re-install Windows)."

After all, they're paying me for the time spent and if it is faster to start over, then that is what I should do, isnt it?