Thank you so much for the information about changing the processor driver in order to deal with problems reaching full speed on a 600X with SpeedStep CPU running XP. Your suggested solution solved my problem in my 600X running a PIII 850MHz under XP SP2.
Having said that, I would note that my testing indicated a number of things not consistent with your results.
According to many users over at the ThinkPad Mailing List, a 600X laptop that began life with a PIII 650MHz installed is able to run at full speed perfectly fine using the original processor driver installed under XP SP2 as well as under a variety of other versions of Windows.
Also, in my 600X that started out with a PIII 650MHz and was upgraded to PIII 750MHz, no processor upgrade was necessary to achieve full speed under XP SP2. This laptop is using the driver file "WINDOWS\system32\DRIVERS\p3.sys" (this is the one that is installed when you select "Intel PentiumIII Processor") and it runs just fine and it changes speed as it should when you change the Power Scheme and/or when you plug in the AC or unplug the AC.
Only on my 600X that was upgraded to a PIII 850MHz did I find it necessary to change the processor driver to the "WINDOWS\system32\DRIVERS\intelppm.sys" (this is the one that is installed when you select "Intel Processor").
Note also that I do not experience any of the system freezing you report, nor is it ever necessary for me to use Fn F2 or any other escape key sequence on boot up, regardless of which processor driver I have installed or which of my upgraded 600X machines I am using. The only noticeable difference appears specific to my PIII 850MHz machine, and on that machine, as you suggest, it appears that this alternate processor driver is necessary in order to achieve maximum CPU speed under XP SP2.
I'm continuing to gather additional information from other users on this issue in other forums and I will post back here once I have compiled my findings.
A record of my testing and exeriences (along with a note of credit to you) has been posted on the Thinkpads.com Forum:
SpeedStep Issues with XP on 600X Upgraded to PIII 850MHz
Thanks again for pointing out the solution of changing the processor driver. I am quite sure I would never have come across this solution on my own.