1. A mainboard, comprising: a central processing unit (CPU); a set-up value memory, coupled to the CPU for storing a group of set-up values; and a BIOS memory, coupled to the CPU for storing a BIOS with an adjusting table and an original set-up default value, wherein the CPU executes the BIOS to make the BIOS read the original set-up default value; adjust the original set-up default value according to the adjusting table to obtain a customized set-up default value; and store the customized set-up default value in the set-up value memory as the group of set-up values and the adjusting table comprises: a flag field, for indicating whether to overwrite a part of the content of the original set-up default value; an overwrite value field, for recording an overwrite value; and an overwrite address field, for recording an address to be overwritten in the set-up value memory.