On a MB that old i doubt it is in BIOS settings.
Look on the motherboard for jumpers.
More than likely there will be setting silk screened on the MB for what the jumpers are for. You need 66Mhz on the FSB and 2.5 on the Multiplier.(i think). Laters...
you might want to clarify exactly what cpu you have (Pentium, Pentium w/MMX, AMD K5, Cyrix 5x86, IDT WinChip...)
PC Support Technician