A computer system has a central processing unit installed inside a housing for processing data and programs. The housing has a panel, which includes a plurality of first holes. The computer system further includes an upper duct and a lower duct installed on a region beside the central processing unit and mounted on the upper duct. The upper duct has a predetermined pattern, and a plurality of first hooks for engaging with the plurality of first holes of the housing. The lower duct has a positioning rib for engaging with the predetermined pattern of the upper duct.