These are good troubleshooting steps to cover the majority of the types of problems. I was taught to gather symptoms and use those symptoms to point to the different subsystems. A common error is to begin diagnosis before gathering enough information. If the problem is intermittent, then you will need more information like how often, does it happen during a particular application and is there something else not working that seems insignificant.

If you get a call after someone else worked on it, get their notes but don't rely solely on that information. Another pair of eyes can sometimes find problems you did not catch, this is a great help for those hard problems when you focus on one part and not able to pull back.

Item 3, is it hardware or software; sometimes a problem has a hardware solution and a software solution; example would be adding RAM or increasing virtual memory. It is best to understand how the hardware works as well as knowing how software works with the hardware. If you only know software then you will tend to blame the hardware when you run out of ideas, and vice versa for hardware experts.

Run a memory test first before defragging. Corrupted memory will corrupt data as the defrag runs. It is probably better to run a disk check or disk test before running defrag.

Experience leads to good judgement, good experience comes from bad judgement. It is important to learn from mistakes. Remember that the majority of tech problems are solvable. Some solutions may be too time consuming and another solution could work quicker; example is to reinstall or reimage the OS or application vs making small changes. If you are stuck on a problem it sometimes helps to eliminate what isn't causing the problem to focus on the parts that are involved.