Computer gets really hot under Linux

When I'm in Linux, even just idling, I can hear the fans of my Pentium 4 (yes, it's a shitty processor, but that isn't the point) working overtime to where they sound like a vacuum.

The only time it does this in Windows is when I have Oblivion or SimCity 4 running for about an hour.

I even have my Linux kernel dynamically-throttle my CPU speed such that Linux only turns up the frequency when a program is running that needs more computing power (I believe this is the CPUFreq series of settings in the kernel setup), and even when the cpufreq is a mere 400mhz, the fans are spinning as if the CPU is going at full 3.2ghz.

What can I do to prevent Linux from making my computer into a sauna-creation machine all the time?

In before "delete Linux and your shitty Pentium 4"

1. Make sure that the CPU is actually clocked down by linux. What facility do you use to achieve this?

One of the in-kernel governors? From your description it sounds like you are trying to use the "conservative" governor. If so, did you make the appropriate entry in your config files so the correct one gets used?

If not, what else are you trying to use? A daemon that dynamically changes the frequency? if so, Did you configure and start it correctly? Did you remember to compile the "userspace" governor into the kernel and configure it correctly?

Watch the CPU frequency for some time as you work. Does the dynamic frequency changing seem to work?
# watch grep \"cpu MHz\" /proc/cpuinfo


2. Your fans running on a high setting all the time might indicate a problem with ACPI in general. Look through your kernel's ACPI config and read a few howtos to compare. Look through dmesg and your logs to see if there's an indication of the kernel having trouble with your BIOS's implementation of ACPI. A number of BIOSes have faulty/buggy implementations. Determine your mainboard/chipset/BIOS model and versions and lookp up on the internet if there's a known problem with ACPI. Sometimes you can find fixed ACPI tables that you can use instead.