A socket A Athlon XP idles around 50c that is normal, they have a thermal spec of about 90c, They are one of if not the hottest running cpu ever made. It won't even become unstable because of heat until around 85c. Compare the temps at idle to your bios temps they should be very close, this will tell you if speedfan is reading it properly. After your install have you installed all the mobo/chipset drivers and do you have the latest bios for your board. What make/model number board do you have?
Never compare your temps to those of a newer Athlon or a P4, They idle close to ambient air temp and raise quickly when loaded.