IIRC its called "Cool'n'quiet" with AMD CPU's and "Speedstep" with Intels.
CnQ needs both BIOS support and a driver installed I think, and obviously a compatible system.

edit, wikipedia actually has a useful page on this:
https://en.wikipedia.org/wiki/Cool%27n%27Quiet

Change the power scheme (Power applet in Control Panel) to "Portable/Laptop". I found a blog entry documenting the different power schemes and their effect on CPU P-states here.

C1E is a C-state or idle-state, it works independently from P-states.

Oh, that's new to me, I thought the only difference in power schemes on XP were the times 'till standby and others you see there. I'll give it a try and report back. Yeah, Cool'n'Quiet is enabled in BIOS and accompanying driver installed. I always found it weird that the only thing CNQ app does is show voltage and frequency, but no other clues.

So there's a manual included with ASUS Cool & Quiet application, it pretty much says it should work if feature is enabled in BIOS, driver installed and Minimal Power Management selected as power scheme. But CPU still runs at max frequency all the time. This computer just doesn't like XP.

Well, I'm testing on 32-bit XP with hacked kernel for support of Microsoft's generic AHCI driver, 32-bit is noticeably more problematic, couldn't get official NVIDIA AHCI driver running either (stock kernel, of course). I can try at some later point if Cool & Quiet works with 64-bit XP.

I booted my old desktop (Athlon 64 X2 5000+, Gigabyte GA-MA78GM-S2H, AMD RS780/SB700) into XP SP3 to check, I'd left the power scheme at "Portable/Laptop", and it does idle at 1GHz (verified with CPU-Z). Oddly, it only idles at 2.6GHz if the power scheme is set to "Always On", I'm sure it used to do that with the default "Home/Office Desk" as well (as the blog post documents). The system does correctly identify that it's powered from the mains.

The XP installation dates from 2008, when it was new, has the AMD AHCI driver and AMD processor driver installed. It did have a Phenom II X2 550BE installed for a few years as an upgrade (until I upgraded to a Ryzen and moved that CPU elsewhere) and IIRC it idled at 800MHz then too.

You can leave OS-based throttling off and use PhenomMSRtweaker or P-ST8 to do the job instead.

AMD Processor Driver was the missing piece (linked on that Wiki page), this comes with newer Windows versions out of box, now changing power scheme in Windows makes a difference and CPU is chilling like it's supposed to. Only took until 2025 'till that was figured out. To be fair, I only daily drove XP for short few months after buying this computer before settling on then new Windows 7 (and then 8.1>10).

Interestingly, ASUS also didn't include that information and didn't package this driver with their CNQ driver or linked it anywhere on their site - CNQ driver shows in Device Manager under System Devices as ATK0110 ACPI UTILITY.

I did try PhenomMSRTweaker, that also works.

It is worth noting that AMD driver's installer adds /pmtimer switch to C:\boot.ini, which likely screws with performance of applications.

Both options don't work well with frame limiting in games. Unless frame limiter is old school, so just busy waits and doesn't give CPU time to breathe, things start lagging massively.

With C1E configuration enabled in BIOS, sound is all rattling on XP. Does anything work right on this wretched OS?