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There's also programs that use the idle thread to put the CPU to sleep when it's not in use: CPUIdle is a good one, even if you do have to pay for it. That's probably a better option if you're running an always-on system like a server.

The author assumed that a notebook CPU runs with 100% load. I have two applications for a notebook: office stuff like writing a message or reading a document, or playing games. Even the latter hardly requires 100% CPU load all the time. For these applications I find a cheap notebook with a software solution like CPU Idle quite adequate. Why spend more money on "mobile" processors? CPU Idle also works fine for desktop PCs.

Actually, the linux box may very well save a lot of power compared to the Windows box because it won't occupy your CPU when it's not doing anything.

As written on the CpuIdle site:

"Under normal circumstances the CPU isn't always active but spends much time waiting for the keyboard, harddisk or CD-ROM. What would be more logical than to turn off the CPU for that period? That's exactly what the HLT machine instruction (Opcode F4) does.
...
Modern operating systems like Linux execute the HLT instruction in an idle priority thread. This thread is always executed when the CPU is otherwise idle. No additional execution time for HLTing is needed, the CPU will not run slower.

While other operating systems like Linux always used this mechanism, Windows only learned it with NT. But even with NT and following versions it is only enabled when the BIOS and ACPI implementation is recognized by the OS."

Basically, not only will Linux keep your CPU cooler this way, it will reduce power consumption since the CPU is literally not doing anything when it's "idle".

I run CpuIdle on my WinXP machine at home and it goes from a normal temp of ~45 degrees Celcius to an average of ~30 degrees, during average desktop usage... Linux will show a similar level of cooling by default. :)

Anyone who uses software powersaving/CPU cooling in windows or linux has heard this noise. Programs like CPUIdle et all put the processor into an HLT state and cool it significantly (12+ degrees here). I run the thing to cool my massive laptop which would get quite hot during renders and things, what with it's 10K RAID etc.. I hear this hum in a lot of electronics that have no moving parts (routers, computers, etc..), and have always wondered about it. In a chat on IRC we chalked it up to electric frequency vibration.

I have also worked on some PCs that make the hum during *any* processor use, like scrolling a webpage, etc..

Get something along the lines of CPUIdle.

I notice a definite change in pitch on my system fans when I go from low to 100% CPU now.
That and the fact that I dropped 10 degrees C on my CPU and mobo temps has to help.

This is true, however note that the 16/32 MS operating systems don't do halt instructions on the idle thread, so they are always running the CPU at pretty much 100%. The NT4/2000/XP series, as well as almost all other multithreaded operating systems, do HLT instructions on the idle thread which effectively "speedsteps" processors. See CPUIdle and the like.