A rack mounted computer system has a vertically elongated cabinet structure in which a specially designed server unit is mounted. The server unit includes a slide-out outer drawer having a power supply box mounted in a lower interior portion thereof. An inner tray is slidably mounted in an upper interior portion of the outer drawer, over the power supply box, and has a system planar board and associated riser cards mounted on its top side. When the outer drawer is pulled out service access is provided to the circuit board apparatus on the inner tray. Access is provided to the power supply box by simply sliding the inner tray rearwardly relative to the opened outer drawer to expose the power supply box. Cooling fans are carried on the inner tray and within the outer drawer to cool the various computer components mounted within the server unit by flowing cooling air generally horizontally therethrough. The cabinet structure is provided with a front access door and a rear panel having ventilation openings therein. During operation of the server unit cooling fans, ambient air is sequentially drawn inwardly through the front access door ventilation openings, horizontally flowed through the server unit, and then horizontally discharged through the ventilation openings in the rear panel of the cabinet structure.