Some CMOS are different.
The common F< key> entries that allow you into CMOS options for ASUS are:
Sometimes < Del> is used on ASUS boards and most of the time F8 is boot sequence settings.
Try F12, if it's not then it's disabled somewhere.
After poking around for a bit I found this:
The firmware hub on the motherboard
stores the Setup utility. When you
start up the computer, the system
provides you with the opportunity to
run this probram. Press DEL during the
POST to enter the Setup utility;
otherwise, POST continues with its
--ASUS P5MT-M User Manual, Section 4-2 Paragraph 3
It seems DEL is the correct key. I can't find anywhere that would disable it. The only physical jumper that would affect CMOS on your motherboard is the clock reset/master reset.
If there's a splash screen, it may be hiding your options, press TAB at the splash screen then press DEL.
There may be a time delay in your monitor that does not allow you to see the options, do you press DEL to enter CMOS even if the monitor is off?