Depends on where are you, what's inside your computer and what time you're using it
The truth of the matter is AMD Athlon XP CPUs were made to withstand high degrees of temperature (after all most if not all Athlon chipsets were made in Malaysia - a country with only a dry and a rainy season). Athlons were tested and were found to function efficiently up to somewhere between 85 - 90C. In fact I had a previous system running at 53 to 55C for 1 year and I didn't encounter any overheating problems.
The problem though is that while the CPU can withstand this much abuse, the rest of your computer's components can't. Another problem: if your CPU fans don't work them CPUs got to over 100C in seconds. That's why some motherboards and cases have temperature sensors that check your computer's inner temp as well as your CPUs and often have automatic shutdown programs that work wheneverthe CPUtemp/chasis temp goes beyond the set temperature.
If you're a bit paranoid about your CPUs temp let me make a few suggestions:
1. Put your computer in a place with good temperature and humidity - like in a room with an aircon or a big fan. When it's hot outside the computer it's 2-4 times as hot inside.
2. Use your computer on the coolest time of the day like in the evenings or early mornings and not during lunchtime.
3. Add some `cooling factor' to your CPU. A thermal adhesive, some better CPU fans, additional case fans, hard drive fans for 7200rpm HDDs can help cool the insides of your computer.
4. Get a bigger CPU case. Switch to a tower case.
Hope this helps