It's not your BIOS changing the CPU speed when idle. Windows is changing the MHz of your CPU when it thinks there's not enough load to justify the CPU running at full speed. You can only disable the hardware that makes this possible in the BIOS, but cannot change any details about this.
What you need to have to enabled or on Auto in the BIOS is "EIST" (Intel Speed Step) for the MHz change, and "C1E" for the voltage change (I think). Then look at Windows, make sure to have 5 % minimum processor state and 100 % maximum processor state set in the Windows power options. The power profile named "Balanced" is set up like that by default. The profile named "High performance" uses 100 % minimum processor state, though I'm not sure that really disables Windows' behavior about putting the CPU to 1600 MHz when idle.
I don't know at what point in the boot process Windows starts to try to keep the CPU slow. I guess it will never slow down to 1600 MHz while booting, as booting has it busy enough for 3600 MHz. It will still use low voltage even at 3600 MHz if there's nothing taxing happening, so don't worry too much about it.
EDIT: This program will show you your CPU's current MHz and current core voltage: http://www.cpuid.com/softwares/cpu-z.html
If you start it as early as possible after booting and getting to the desktop, I think you'll see it'll be at 3600 MHz for a pretty long time. Eventually the HDD light stops flickering and booting will be finished, and then you'll see 1600 MHz appear.Edited by deepor - 4/14/13 at 8:03am