1. A method for conserving system resources of an electronic system with a BIOS, a first memory unit and a second memory unit, the electronic system being connected to a detachable card with a plurality of slots disposed thereon for receiving a plurality of slot cards, the method comprising the steps of: (1) allowing the BIOS to perform a POST (Power On Self-Test) program; (2) determining whether data in the first memory unit is invalid, if so, going to step (3), else, going to step (4); (3) determining an identification code of the detachable card, so as to determine and record bridges corresponding to all the slots on the detachable card in the first memory unit; (4) enabling the bridges that are recorded in the first memory unit; (5) allowing the enabled bridges to communicate with buses that are controlled by them, and determining width of the corresponding buses; (6) determining an identification code of the detachable card, so as to determine the bridges corresponding to the slots on the detachable card that are inserted with the slot cards and record these bridges in the second memory unit; (7) determining whether the bridges recorded in the first memory unit equal to the bridges in the second memory unit, if so, proceeding to step (8), else, going to step (9); (8) allowing the BIOS to perform a Plug-and-Play (PnP) algorithm so that the system resources are assigned to the buses with the enabled bridges, and then, ending the method; and (9) overwriting the bridges in the first memory unit with the bridges in the second memory unit and resetting the electronic system, then returning to step (1).