Windows 3.1 Serial #

My friend just picked up an old PC that has Win 3.1 with the Win 95 Update. The PC has a small HD that has been filled up, now the PC is extra slow. He wants to format and reload but we cannot find the serial # for his Windows 3.1. Does Windows 3.1 have the # on the HD? The system is still up. Or does someone have a generic # that would work?