Intel SpeedStep, Windows XP, and confusing Power Profiles

This information is elsewhere on the net, but not necessarily easy to find, so I’m “re-documenting” it here while I still remember it from my work in this area back in 2002-2005. These states are mapped for Windows XP terminology only, but a great deal of IT machines are still out there that use XP so it’s probably still relevant for another year or so. The impetus to write this down was due to my wife’s work laptop “being loud all the time”. I took a look and discovered that it was her fan that she was complaining about. Since it had the Centrino Mobile Technology sticker, I knew the processor had to be Pentium M or later, fully supporting Intel Enhanced SpeedStep Technology, so I changed the power profile to allow the CPU to enter a low power state while the laptop was plugged in (which is almost all the time). About ten minutes later, everything was perfectly quiet (the fan having stopped running for the first time).