replace the whole heatsink and fan with a low noise one. I now have a virtually silent Arctic Cooling hs/fan on my Athlon XP 2100+, cost about a tenner from Ebuyer.
Spins so slowly I had to disable the motherboard cpu fan speed warning.
just make sure there's room around the cpu socket for the heatsink cos they tend to be larger than the AMD OEM versions. Also be sure to remove the old and apply new thermal paste between the cpu and heatsink. My fan / hs came with a small tube included; some have thermal compound already applied to the base of the heatsink.
And be careful with the screwdriver when releasing / reattaching the heatsink securing clip. You sometimes have to apply quite a bit of force to those Athlon clips. I found removing the PSU gave me much better access since my cpu socket is right next to it.