Its probably the Slot 1 mounted "Klamath" P2, and should have unlocked multiplier. Overclocking through multiplier is safer on old systems as raising FSB will in most cases raise PCI and AGP bus frequencies too, which may result in damage to components other than CPU. If your motherboards BIOS doesnt offer any cpu options then the motherboard might have a set of jumpers for FSB and multiplier settings(well, it pretty much should have one or the other). Do you know what motherboard is it? Inspect it thorougly first, because pin mod on that cpu(if possible at all) wont be no simple pencil mod but will most likely involve soldering resistors onto cpus pcb.
And for comparison - I once installed XP on an old laptop with 400mhz cpu and 128mb ram - it ran horribly slow