You say Athlon which I think you have a Athlon XP model so are you saying you have a Athlon 2200XP which is running at a 1.8GHz (Which is normal)
Is you Motherbaord new or have you upgraded the processor ?
Do you have the Option for Auto for the CPU setting in BIOS if yes use that is best bet. If you do not then set it at Manual so the CPU if a XP2000 (1.8GHz) should be set at 1800 for the CPU.
I am a bit cautious on giving you this advice as I prefer to know if this is a old or new M/baord and don't want the responsibilty to fry any chips.