A plenum for guiding a flow of cooling air through the interior of a computer is designed to maximize the airflow over those electrical components within the computer which are most sensitive to temperature, and/or which require the greatest amount of cooling. The plenum is designed to be mounted over top of a computer motherboard which includes a plurality of electrical components mounted thereon. One or more apertures are formed in the lower wall of the plenum so that electrical components mounted on the computer motherboard can protrude through the apertures and into the interior of the plenum. The lower surface of the plenum is designed to prevent the flow of cooling air passing through the plenum from escaping through the apertures on the lower wall. Also, upper and lower protrusions on the plenum can extend into the interior of the plenum to concentrate the flow of cooling air onto the electrical components which are protruding through the apertures and into plenum.