I'm not sure if the 2 things you are asking are one in the same. The fan is about the only way to regulate the temperature of the CPU. Faster fan speeds make things cooler, slower speeds make it hotter. There are HSFs that can monitor the temperature of the heatsink, and adjust speed accordingly. As far as I know, there isn't software that allows you to do this manually.
The only other way is to wire in a voltage regulator to your heatsink fan. Reducing the fan voltage to 7V will slow the fan, but raise temps and lower noise levels.
Just be sure that you have a fan running at all times over an XP2200. I don't want you losing your investment. You may also want to go into BIOS and setup a "Shutdown temperature" for your CPU (found under PC health). Hopefully that will save you if something goes amiss.