As a computer technician, you'll often run into the dilemma of either staying and continuing to work on a problem that you don't definitely know you can fix, or reformatting the hard drive of a machine to eliminate the problem once and for all. Let's talk about when to reformat the computer, for computer technicians.
The first scenario is when you have a problem with startup, and the recovery or Windows disk is not able to help you. This can be the case in many situations. The typical reason the startup disk will not work in repairing the computer is that the actual files within the operating system that allow that fix to work, are damaged in some form. That will make a computer unrecoverable. That's when you know it's time to pull the plug, get as much data off of the drive as possible by using it as a slave (attaching it to another computer through a USB tool), and rebuild the machine.
Another scenario that you could run into is the when the files are missing from your start menu and programs. This can happen as a result of damage left by some of the latest fake antivirus viruses. It makes the user think that the files are gone, and the programs do not work anymore when really it's a matter of unhiding the files, and changing the paths for certain start menu items. If you are going to attempt to fix this problem, I would highly suggest using a program called "unhide" which can be found at bleepingcomputer.com.
If you cannot get the files back in the right place, and you're racking up the time, it's time to reformat. The challenge with being a computer technician in these situations is that you, because of your curiosity and maybe pride, want to figure out how to fix the problem. You don't want to get beat by some virus. But the cold hard truth is that you may be wasting your own time trying to fix something that may take you 5 hours, while you could just make the decision to back up the data, and reformat. Clients will very rarely be ok with paying you five hundred dollars for a single computer repair service call.
Overall, the best rule of thumb is to make the cal to rebuild the machine once an hour of your time has been spent in troubleshooting, and you still don't have a solution, or a time frame in which a solution can be found and a fix applied. Now, you still have time to reformat and keep the repair under 2 or 2 1/2 hours, you'll be saving yourself some time, and you'll be saving the client money.