You'll need to get that case open if you want it to cool better or replace a fan.
How old is your machine? Maybe the fan doesn't run as smooth as it used to. When it's open spin it with your finger and check if there is any significant resistance.
AMD does run somewhat cooler than Intel (at equal performance levels excluding mobile chips), this is about to change, but that's besides the point.
If you have an older Athlon XP without the headspreader it is very possible that your heatsink has moved or the thermal paste has dried up. I got a relatively heavy block and i've seen cpu temps magically rise after going to a lan, then go to normal after i've reseated the heatsink with some new paste.
You can buy some paste yourself at any local store (you can get all sorts but expensive ones have little added value for ordinary users and may be difficult to handle), and look up a guide on the internet on how to mount a heatsink with paste on an Athlon XP or a Pentium-III, as they are similar. If you like to play it safe, just bring it in and have your problem sorted, but you will need to pay the bill.