Sup buddy. Maybe the ECS K7S5A motherboard won't support the XP 2100+ even with the latest BIOS. Tried leaving it on default settings in BIOS and then thoroughly formatting the hard drive before installing Windows XP? What exactly is mentioned in the error message?
Btw, I finally upgraded and now I'm expecting my Maxtor 60GB ATA133 7200rpm hard drive 2moro!
Could be a possibilty since the fan you are using might not be approved for processors such as the AMD XP 2100+. Make sure that you are using a good quality fan/heatsink such as the Coolermaster ones. I believe that there is one on ebuyer which supports up to the AMD XP or MP 2100+ but I would suggest getting the one which supports up to the AMD XP 2700+. I think it has some speed adjustment option on it as well.