XP doesn't have BIOS memory at all. So there's no possibility to disable it, so no need to do that. Either that someone doesn't know what he talks about, or you misunderstood it.
The only proven way to make a PC fast again: system recovery back to factory conditions from your system recovery disks or partition.
My Dell desktop has become so slow that it sometimes freezes. Someone suggested I disable the BIOS memory, but I do not know how.