English is often unclear about past imperfect tenses being clearly expressed as imperfect, but here we use "had a computer" as a past continuous (imperfect) because people keep their computer for 2 years or more. If we chose a verb whose meaning conveyed an act that was punctual, inchoative, temporary or quick (e.g. to buy a computer) then we would use the pluperfect as you suggest ("had bought a computer").
Do you see my meaning?
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