This is analogous to a mechanic spending an hour inspecting a system and then tightening one nut. The lay-man may look at the mechanic and think that the job was easy as it just involved tightening one nut. However, it is the fact that the mechanic knows which nut to tighten that makes their service valuable.
There is also a fair bit of inductive reasoning involves in computer repair. If you have a virus that only activates when you connect to a network, and you test this 5 times, you can conclude that that the virus is being activated by the network. Of course, it could have been sheer coincidence that your virus activated 5 times in a row precisely when you connected to the network but as with all scientific pursuits we must draw the line at a reasonable level of probability that there is a causal link between two events.
After we have abducted (past tense of abduction?) our theory, we test our hypothesis by applying the relevant fix. If our hypothesis as to the cause of the symptom was correct, then the computer is now fixed. If the computer still has problems, its back to our logical drawing board to develop a new hypothesis.