PS/2 to Serial and USP to PS/2 adaptors for mice are specificially made for that model mouse, and nothing else. Essentially the mouse can support either, and will run the right signal down the cable depending on what it detects. There is no actual smart component in the adaptor it's self, thus the adaptors cannot be combined.

Look up the motherboard. Being a Pentium class board, I bet it has a PS/2 port, it's just going to need a cable with a PS/2 connector to be plugged into the header on the board. To easially verify this, check the BIOS for any PS/2 options, or check the Device Manager for a PS/2 port (it usually uses IRQ 12).