Just a note, but ACPI is not the same as a HLT command. ACPI will lower your idle temps pretty well, but not to the degree of a cpuidle, or other program that uses a windows HLT command.
Since WinME and Win2k use a hlt command automatically, the only time you need a program for either of these OSs is when you're using a dual-cpu setup, in which case the win2k HLT command doesn't work.
Losty,
If you go to your windows task manager(winME/Win2k), and go to the processes tab, you'll see something called "system IDLE Process". This is the hlt command/exe.
Mike